There are several pen pal matching organizations for military service members, such as Adopt A U.S. Solder, Adopt a Platoon and Any Soldier. Each of these organizations has a website with easy sign-up information. Some encourage sending care packages as well.
Any Soldier has an extensive FAQ page explaining what to write, what to send and how the correspondence is handled. Any Soldier began in 2003 when an Army soldier in Iraq asked his parents to send even more care packages than they had been sending because there were so many soldiers who never got any mail. The soldier's parents began sending extra letters and packages addressed to "any soldier" that their son would then distribute to other soldiers who did not receive mail.
Operation Gratitude encourages many ways of showing appreciation for service members, including writing letters of encouragement to those serving. Letters are needed both for actively serving service members and for veterans and wounded warriors. Adopt A U.S. Soldier is another nonprofit, volunteer organization that is active in over 170 countries with over 1,000,000 active volunteers. Adopt A U.S. Soldier encourages its volunteers to write to their adopted soldiers at least once a week and develop a long-term commitment to supporting the soldier with letters and postcards.