What would you bring on deployment?

Get a 5 gallon bucket from Home Depot. It'll fit in your duffle bag. During OIF I I used my bucket to do laundry, used it as a chair, and even mixed concrete in it when I was doing position improvement.

X2 undergarments, doing laundry is a pain. Anything that you like to do as a hobby, to keep you occupied when you are not at work. For me an iPod sufficed, skype email and internet, but that all depends on where you are going. Also twice the toiletries you like should go in the box. Let the care packages bring snacks and whatnot but do not. Rely on them for your preferred deodorant.

Drink mix (kool-aid, Gatorade, tea) because only having water to drink gets old quick. Cash, because you probably won't have access to an ATM but may want to buy something. Baby wipes, lots and lots of baby wipes. A bucket and laundry powder to wash your clothes. Air freshener, to spray on your IBA, other gear, or to spray on the nasty body who doesn't clean himself. A camping shower. Thick shower shoes because there's not many things worse than standing in nasty shower water. Toilet paper. Extra razors, shaving cream, toothpaste, toothbrushes, t-shirts, sox, and at least two laundry bags. Decks of cards (playing and Uno). Pens, envelopes, paper to write letters (you don't need stamps because you can mail them home for free). Snacks that don't melt or crush, think beef jerky

Since everyone is giving the "right" answer, I'll give you another "right" answer. Porn. Laugh till it hurts, but let a HD pop out and see how much you can trade for what's on it, get additional material, or if you're heading to the Stan somewhere, it's really amazing what the terps will get for you just for you to "free the nipple". I learned that from outgoing grunts when I was incoming. I got a 1TB HD and all hell broke loose for the next year. Never left home without it.

This is easy -- my loaded-up Kindle.

Baby wipes; a coffee maker, preferably not glass; extra canteen cup; books; decent snacks to dunk in the coffee; extra poncho liner; a pillow

Have done many of these, as we are the first ones in to start building up the areas.At first I use to take every thing I could and never used half of it. In mountains of Pakistan, we had a small camp the had nothing, at all, we were lucky to have a SOF COMS element with us out of Germany, so communication,as in internet, never went down. I would bring a serge protector, Extra Batteries, depending on location and environment and your mission, extra socks, cold weather under gear, other than issue, you can never have enough of. I would take some bed sheets, as they may have bunks, not cots, and the mattresses, sheets will not take up space. The biggest thing I would take extra of would be Baby wipes, and more Baby Wipes, and more Baby wipes. When we deploy on our own or isolated area, we take a Laundry IS 90, with laundry Machines and Dryers, and everyone eles loved us. My point to that story is check on laundry, and you may want extra powder detergent, to do it yourself sometime, If you put in your box Make sure to bag it well, customs, and Force Protection Might no like it if it leaked. The other is extra Gym shoes, and shower shoes, with a positive attitude, and if you have Engineers with you, get to know them well, Engineers can always make roughing it better. I mean Combat Engineer Types, Seabees and or Red Horse.

I never leave without a heating blanket. Some places get really cold or I am always the only cold person in the room.

Baby wipes for cleanliness. A deck of cards. A good book or two or three. Something that you won't mind reading again if you blow through it really quickly. My favorite is the Ultimate Hitchhiker's Guide to the Galaxy because it's funny, it contains 5 books and is like 1000 pages. Nice way to kill time or get a laugh.

Adaptability. Combat environments are constantly changing and so will you. "Your environment will never adapt to you so you must learn to adapt to it." I heard someone say this once.