Dunno about sending to people you know but if you want to send packages to more random personel
AnySoldier.com has guidelines. You can also find people to send packages to sorted by various factors such as branch of service, gender, home town etc..
I send
CD's
DVD's
Video Games
Books
Magazines
Buy bulk candy and snacks and re-bag in smaller portions (zip locks)
Wipes
Drink Mix-esp in single serving sizes
Disposable cameras
Phone Cards
paper, pens, envelopes
Socks
Shorts
T-shirts
tooth brushes
just about any item from the drug store
batteries
toys and things for the kids
Liquor (may get them in trouble-check first)
DO NOT SEND ANY REQUEST FOR A FAVOR IN RETURN.