Baby Wipes - packets are better than tubs. Much better than baby wipes, per se, are the "Hoo Ahhs" brand of pre-moistened, alcohol-free, unscented Field Towels
Beef Jerky/Slim Jims
Energy Bars (Power Bar, etc. - Unless a specific brand is requested, much better than standard civilian energy bars are the "HOOAH!" brand energy bar based on the U.S. military's own specially formulated bar that better meets the soliers' needs)
Hot Sauce (Tabasco, Red Devil, etc.) - wrap glass bottle in duct tape and place in doubled zip-lock bags
Sunflower seeds
Peanuts
Seasoning Salt (Lawry's, etc.)
Atomic Fireballs / Jolly Rancher hard candy
Powdered Energy/Electrolyte Replacement Drink Mix (Gatorade, etc.)
Chewing Gum
Avon Skin So Soft - great for removing face paint and also reputed to be an effective bug repellant (the latter seems variable based on personal experience and reports)
AA-cell Alkaline Batteries
Personal Hygiene Gear: Disposable Razors, Toothbrush and Tooth Paste
Eye Wash / Drops (Visine, etc.) for flushing dust and sand out of the eyes
Zip-Lock style Plastic Storage Bags - assorted sizes, heavier freezer style are better
Copenhagen Snuff in a tin ("Even if the service member doesn't dip snuff, they will be a hero and have more power than the Commanding Officer if they have a 'log' (10 cans) of 'Hagen.' A large percentage of infantry use this stuff and it is hard to get. The stuff they do get comes in a plastic can and tastes terrible. The 'Stateside Hagen' is the most powerful bargaining tool in a field environment!")
Pre-paid Phone Card so service member can call home
DO NOT SEND:
Anything that melts (if you just must send chocolate, send M&Ms)
Waterless Hand Cleaner/Sanitizer - generally makes mud, wipes are preferred
Flashlights - Everybody has one and never gets to use it due to visible light discipline restrictions.
Compass - Everybody SHOULD already have one attached to their Load Bearing Vest/Harness
Sun Screen - already issued in mass quantities, drowning in the stuff
Liquor of any sort
Pornography of any sort. This includes "soft porn" such as Playboy magazines.