• universal power adapter - helpful not just for the country you’re going to, but countries you’re traveling through. I forgot to bring this once, and was shocked at how hard such outside the USA. 
• two flash drives - to back up your photos, essential documents, reservations, etc.
• earplugs - take more than just two
• ExOfficio™ underwear - or a similar brand, that you can wash and will dry overnight. Many places where you stay will either require you to wash your own clothes, or want you to pay for the services of a local person to do this - but that person probably won't wash your underwear. Using ExOfficio or a similar kind of underwear allows you to take just a few pairs that you can wash in your sink or a bucket every night. For a six month deployment or less, three pairs is probably all you need.
• sink stopper - I joke that I could take several drain plugs when I work abroad and sell them for $10 a piece to fellow aid workers, as so many of my colleagues forget this. If you have to do any laundry in a sink, or if you want to take a bath, you will need this. The ones built-in to bathtubs and sinks abroad rarely work.
• door stopper. When you take it out from under the door to go out that door, put it in your purse or somewhere such that you absolutely will not forget it when you pack and leave.
• Parmesan cheese packets - if you can’t get it where you’re going, you will be so glad that you brought this. Will make any bland dish taste better.
• medications - any prescriptions you need (be sure you have all documentation for such), anti-diarrheals, antihistamines, stuff to address or prevent heart-burn, prescription treatments for a yeast infection and/or UTIs, any asthma medications/inhalers, and large bottles of aspirin, Ibuprofen, and acetaminophen. You can get all of these medications in any pharmacy abroad, but it can be very hard if you don't speak the language, the medication may be expired, and it's very hard to find large quantities of aspirin and other pain killers abroad.
• first aid kit (and know what is in it!)
• face masks: in this era of global pandemics, you need to bring masks. Each should have a nose piece that you can adjust to fit snugly over your nose and that fits relatively snugly against your cheek and under your chin, and goes high up on the cheeks. An N95 is best, but those can be very hard to get. I use the Outdoor Research Face Mask. It comes with a removable inner filter (sometimes, I put two in), I can wash it, all are dark colors (doesn't show dirt), and they all fold well. This, sometimes in addition to my headband around my neck/neck gaiter, provides me with the protection I want and keeps others safe from me and whatever I'm breathing. A lot of travelers got stuck abroad when the novel coronavirus broke out without one (including me).
• portable smoke detector - if you might be staying in a place that's above the second floor in a developing country, you will want this. There's one available from the ILS store (many of the items listed here can be found there as well).
• portable carbon monoxide detector - when I worked at UN Volunteers, one of our volunteers died in the field because of carbon monoxide poisoning in his guest house. I once read about almost everyone dying in a guest house in Spain from carbon monoxide poisoning. 
• batteries for your portable smoke and carbon monoxide detectors (you can buy these in-country, probably, but go ahead and have them loaded with fresh batteries before departure - and make sure the airlines you will take allow such batteries to be on the plane)
• portable water purifier and filter replacements - you can find these at most any outdoor store, including fishing and hunting stores. 
• multi-use tool - but remember that you cannot carry this on an airplane, because it has a knife - it has to be with your checked bag! Get something not only with a knife and little saw, but also a cork screw, bottle opener and can opener.
• hidden purse/money belt to wear under clothes - big enough for your passport. 
• cash stash keyring - but don't keep it on your keys; keep it on your bra or in another absolutely secure place on you
• undercover bra stash bag - keeps money in a very safe space under your clothes
• keyring flashlights - I cannot believe how handy these are! 
• head lamp - great for late night trips to the bathroom, essential in an emergency when you need to get somewhere - like OUT - in the dark
• small padlock - I use these to lock zippers together on my bags that I'm carrying in public.
• metal door lock - offers additional security when inside your room. There's one available from the ILS store (many of the items listed here can be found there as well).
• cloth bags or any folding backpack or duffel bag. You will often need to transport things between your guest house and work site. This is a great way to do so. And you can leave these in-country when your mission ends; your local colleagues and their families will very much appreciate such (you will note how valuable these are when you see so many people transporting valuables in plastic bags)
• mosquito net - even one that just goes around your head (not essential for every location)
• sleeping bag liner - great for sleeping in between scary sheets and blankets. Packs very small.
• Lonely Planet, Rough Guide or other brand of travel book for country to which you will be living. And be sure to read it BEFORE departure, especially the sections regarding safety, history and culture. I've had some aid workers mock me when they see such a guide on my desk ("Oh, so you're a tourist here, not an aid worker?!") - and later ask if they can borrow it.
• pocket dictionary for country where you will be living (and always have it with you). 
• a wax, also known as a sarong or sarung. A wax is a large piece of colorful but durable cloth that is often worn by women in developing countries as a wrap-around skirt or very large shawl. You can do the same, or use it for a top sheet when your sleeping bag or the European-style comforter is too much on a warm night, as a head dress (often needed to tour a mosque or conservative Christian church), extra covering in conservative countries, a picnic blanket... You can buy them at any hippy/dippy store, any import store, and the like in your home country. They pack very small -- I can easily fit one into a fanny pack WITH other stuff. If you are a size 12 (USA) or a or less, you can use a wax as a skirt as well, even over pants. If you're going to be taking a particularly long trip, take two (so you can use them both at the same time -- skirt and shawl, skirt and headscarf, etc.). Wash them once at home in cold water, to see if they shrink (you don't want to take one that shrinks!).
• a multi-functional headband, one of those long, seamless tubular things you can wear in different ways (a headband, a bandana, a beanie, a pirate-style cap, a neck scarf, neck gaiter, etc.). It's great for bad hair days, when your neck gets cold, or when you are in a very religiously-conservative country where your head covering must cover all of your hair (put on the headband so that it covers your hair, particularly in the front, then put your wax over your head as a head scarf - voila, you're culturally sensitive!). You can find them at most outdoor stores or motorcycle shops. And if you wear it around your neck at all times, you have an on-demand face mask. If you wear this over your face mask, you have excellent protection against the novel coronavirus or whatever the cause of the latest pandemic, plus you protect those you are supposed to be helping from whatever YOU are breathing out.
• cloth bags. you will need these to transport groceries, dirty clothes, toiletries to and from a communal bathroom and on and on. I bring super cheap ones, like bags from conferences I've attended, especially if they fold up really well, and sometimes, I leave them in the country where I've worked, giving them to cleaning staff or co-workers (who are SO appreciative). FYI, some countries do NOT allow ANY plastic bags - not even in your luggage that you bring into the country!
• don't take electronics, jewelry or other valuables while traveling that you would be heartbroken to lose, or that are easily broken. If you are traveling by plane, then keep any of these items, along with documents, medicine (with copies of prescriptions) or any essentials, in your carry-on bag.

• prescription treatments for a yeast infection and/or UTIs
• shirts that are durable, easy to hand wash and extend down over your hips, farther than the bottom of your butt,  that do not expose any of your cleavage whatsoever, and are not at all transparent. For most countries, it's acceptable that these be short-sleeved and even somewhat form-fitting - check travel web sites regarding local culture, consult a guidebook for the region (see aforementioned recommendations), and look at photos online from the country, to know for sure regarding what clothing is acceptable. Even if shorter shirts are acceptable in a country, take a couple of these more conservative shirts for more formal occasions.
• hiking skirts are awesome, because they are especially durable, stain-resistant, easy to wash and dry, and often, can go from full-length to knee-length with just an unzip. And with leggings, a summer skirt can be worn in winter.
• whether it's a three-week or three-month deployment, I like to take three one-color, dark-colored skirts (black, gray and brown). I prefer floor-length skirts, because they are modest - I travel to a lot of countries where modesty for women is an unwritten rule (and sometimes a written one) - and because skirts look more formal (I work in offices and, if outdoors, at events). I prefer dark because they hid stains better. Just three gets me through a deployment of up to six months. My three favorites weren't bought at a camping store - I bought them at a local grocery store here in the USA that has a large clothing section, believe it or not. If you're going to be working outdoors doing highly-physical work (shoveling, climbing on roof tops, etc.), three skirts probably aren't necessary, of course.
• if pants are acceptable, hiking pants are awesome, because they are especially durable, stain-resistant, easy to wash and dry, have lots of pockets and often, can go from full-length to shorts with just an unzip. Sure, it may further identify you as a foreigner, but the reality is that you need to be comfortable and not worry about getting laundry done constantly.
• leggings to wear under skirts, especially in religiously-conservative countries or under skirts or pants for cold weather.
• silk long-sleeve undershirt (can make any outfit warmer as the weather turns colder) and, perhaps, long underwear as well, if the weather might be cold.
• try to take just two, maybe three, pairs of shoes, depending on your length of deployment and your work setting. Any shoes should be super comfortable for you to stand for hours or walk for hours. In Afghanistan, I took hiking boots and Teva sandals - I worked mostly in offices, but did go into the field a few times, and the ground I walked on ranged from snow to deep mud to dirt roads to gravel to polished floors. In Ukraine, I worked in an office, so I took three shoes: hiking shoes for weekends and to get to and from work, Teva sandals for the same when it was too hot for those shoes, and black pumps for low heels, which I kept at work and changed into each morning and wore around the office.
• I like hiking bras when I work abroad: they are durable, they wash well, and they are ready for whatever my work day might hold.

Consider the climate and environment of where you are going - is it generally hot? Cold? Rainy? Dry? Pack for a variety of conditions, of course, but do research about what weather extremes you might experience in the country. Remember: air conditioning is expensive and even if you are going to work in a government office, they may not have air conditioning. Be prepared to layer.

These recommendations will let you look at least a little different every day, so you don't look like you are wearing the exact same clothes day after day. And that's important because, worldwide - and I HATE this - women are judged by how they dress. Women in even the poorest of countries take a great deal of pride in how they look and the image they present, especially with foreigners, and they expect you, the visiting aid worker, to as well - they expect you to respect them by doing so. So, no, it's not weird to take make up when you go abroad, as in some countries, a woman not wearing makeup is a sign that she's a slob and not worth listening to. It's not weird to not want to look like you are wearing the same clothes if you are going to work in an office setting because, again, local women will see pride in your appearance as a sign of respect for them. Have a look at the web sites of NGOs in the country and look how local women dress in official photos - that will give you an idea of what's expected of you, especially if you will work in an office setting. 

On my first overseas deployment, I checked TWO bags and had TWO carry ons - and now, I realize I over packed. On my latest overseas deployment, seven years after the first, I checked ONE bag, took one carry on to fit in the seat in front of me and a large bag as a purse and computer bag. I can get all the aforementioned on my packing list into that, believe it or not. 

Is it appropriate to buy and wear local clothing? This is a frequent debate among many. In Afghanistan, I asked two local co-workers, separately, and two expat co-workers, how it would be perceived if I wore a shalwar kameez I bought in India, and they all gave me the green light. In Ukraine, I asked co-workers how it would be perceived if I wore a traditional Ukrainian blouse, and they were so excited I wanted to that they took me shopping. But some locals may not be cool with you showing up in their clothes - they feel it's cultural appropriation. When in doubt, ask. And remember that two different people may have two different opinions.

• Buy tough, boring-looking luggage. Go for durability, toughness, and ease-to-carry/roll around/carry, not for fashion, because your luggage is going to take a beating, and needs to be able to withstand such. Also, the more expensive it looks, the more likely it, or its contents, will get stolen.

  A large, soft-sided, wheeled backpack, with zip-off day-bag, is, in my opinion, ideal. A soft-sided, wheeled backpack with VERY strong zippers and that allows you to pull the bag most of the time, and to throw it on your back as needed, is also good. Your back will love you for pulling the bag on wheels most of the time rather than wearing it. Make sure those wheels are durable, for the times you will walking on very unforgiving sidewalks. It's up to you to decide if you want to buy a bag you know you can usually carry on (small planes are very limited on the size of bag they can allow to be carried on) or if you also want a bag to check.

  Make sure you take only the number of bags you can carry or drag by yourself for at least five blocks, even a quarter of a mile. I believe that you absolutely should NOT bring more bags than you yourself can carry up and down 15 steps and at least five blocks entirely by yourself. You cannot transfer quickly from one train or bus to another by yourself if you are struggling with bags -- and it also makes you a prime target for thieves. You cannot count on someone helping you in the airport or a train station as you struggle with your bags. And if you get on a train and have more bags than you can handle, other passengers will NOT be nice to you - in Europe especially.

  One piece of your carry on luggage should be the largest allowed for the seat in front of you in economy class on a plane. On full flights, if you are one of the last people on the plane, there will be no room in the overhead compartments by the time you board for your carryons; in such circumstances, if you have just one bag that fits in the seat in front of you, you won't have to check your bag (so long as this is the only bag you are carrying on).  

• Remember to budget space in your luggage for things you want to buy -- clothes, small ceramics, textiles, art, etc. You can buy another piece of luggage in your host country if you need more room - you will have to pay for that extra bag to be checked, but maybe that will be worth it to bring unique items you've purchased abroad. Also, to make more room, consider leaving things you brought in-country for other aid workers if you really won't need those items back in your home country.
  
  
• I tie some multi-colored yarn around the handles of any luggage I'm going to check, so they are easier to spot on a luggage conveyer belt.
  
  
• Your checked AND carry-on luggage needs to have a name tag on the outside, and identification inside -- it's best if the inside i.d. is in more than one pocket of your luggage. Include your name, email address, phone number and physical address, both where you are coming from and where you are going to. If you will be traveling to more than one location, leave a copy of your trip itinerary inside your luggage so you can be located more easily.
  
  
• Airport security will no longer allow locked bags of any kind, whether its checked bags or carry-ons. If you do lock your bags, your locks WILL be broken. That said, do pack some little locks, and use them to lock your bags AFTER you leave the airport, or after you board the plane (yes, people steal things on planes).

A good twitter account to follow on the subject of packing for travel abroad: DepartSmartOrg.

The CDC Yellow Book: Health Information for International Travel, last updated in 2020, is published every two years as a resource for health professionals providing care to international travelers. The fully revised and updated CDC Yellow Book compiles the US government’s most current travel health guidelines, including pretravel vaccine recommendations, destination-specific health advice, and easy-to-reference maps, tables, and charts. It's available free online. Note that Chapter 9 is all about health care for aid workers traveling abroad. As the site notes, "Humanitarian service can have an adverse effect on personal health. Studies of long-term humanitarian workers indicate that >35% report a deterioration in their personal health during the mission." Have a read if you want further guidance on what to pack. It also has advice for the Business Travelers, for Aircrews, for Long-Term Travelers & Expatriates, for Study Abroad & Other International Student Travel, for Adventure Travel, and more (including advice for Sex & Travel). Note: the 2020 version was published before the COVID-19 pandemic.

Also see

12 Reasons Not to Volunteer Abroad
These are the most common reasons people say they want to volunteer abroad. And they are not good reasons. In fact, they often hurt people and animals in other countries, rather than helping.

Volunteering To Help After Major Disasters.

Volunteering with organizations that help animals and wildlife.

Ideas for Funding Your Volunteering Abroad Trip.

Tax credits for volunteering (for residents of the USA)

How to Make a Difference Internationally/Globally/in Another Country Without Going Abroad

Using Your Business Skills for Good - Volunteering Your Business Management Skills, to help people starting or running small businesses / micro enterprises, to help people building businesses in high-poverty areas, and to help people entering or re-entering the work force.

Helping People Address Their Problems with Plastic
How to mobilize a community to clean up plastic bottles, plastic bags and other plastic waste from their environment, and how to reduce their use of such items in the future

Group Volunteering for Atheist and Secular Volunteers

Details on how to quickly fill a community service obligation from a court or school.

Ideas for Leadership Volunteering Activities
These are more than just do-it-yourself volunteering - these are ideas to create or lead a sustainable, lasting benefit to a community, recruiting others to help and to have a leadership role as a volunteer. These can also be activities for the Girl Scouts Gold Award, the Duke of Edinburgh's Award (U.K.), a mitzvah project, or even scholarship consideration.

Ideas for Creating Your Own Volunteering Activity.

Donating Things Instead of Cash or Time (In-Kind Contributions)

Careers Working With Animals (for the benefit of animals)
You are NOT too young to volunteer! Ways you can volunteer, no matter how young you are

Finding Community Service and Volunteering for Teens

How to Find Volunteering Opportunities, a resource for adults who want to volunteer

Creating or Holding a Successful Community Event or Fund Raising Event.

Fund Raising For a Cause or Organization

How you can advocate for an issue important to you

Volunteering with Seniors.

 
 
 
Suggested books:

 
How to Live Your Dream of Volunteering Overseas

Lonely Planet Volunteer: A Traveller's Guide to Making a Difference Around

Alternatives to the Peace Corps: A Guide to Global Volunteer Opportunities, 12th Edition

 
Volunteer Vacations: Short-Term Adventures That Will Benefit You and Others

 
The 100 Best Volunteer Vacations to Enrich Your Life

 
Volunteer: A Traveler's Guide to Making a Difference Around the World (Lonely Planet)

 
Frommer's 500 Places Where You Can Make a Difference

 
The Insider's Guide to the Peace Corps: What to Know Before You Go