I'm heavily involved in a project, PM me if you need any help. It's much easier to go through an existing program than start your own.
Just remember that many items cannot be shipped without causing problems.
The National Guard angle is the best. As BP said, it allows you to add a local flavor be it potato chips, pretzels or any other local delicacy. If no local Guard units are currently active, the DoD will give you the units of local active duty personnel. It's a huge deal when one guy gets so many care packages that he's forced to share....makes him the most popular man in theater.
Far and away the most popular item is phone cards, followed surprisingly by letters. Letters with even the most mundane items take time to read and killing time is always a huge issue.
If foodstuffs are what you're looking at, beef jerky as Hos said is popular as are those little individual cereal boxes that come in the mixed eight packs. Don't try chocolate BTW, it tends to melts in transit.