I'm not sure if this has been discussed before, but I'm sure there are some on this forum who are unfamiliar with it so want to spread this information. The Red Cross is sponsoring an effort called "Holiday Mail for Heroes," that allows people to send Christmas cards to members of the US Armed Forces. I remember the days of simply sending a Christmas card to "A wounded soldier" at Walter Reed, but sadly those days are over thanks to heightened security. Thankfully, a fried of mine whom works for the local Red Cross chapter let me know about this. The address to send it to in the USA is:

Holiday Mail for Heroes
PO Box 5456
Capitol Heights, MD 20791-5456