Last October and again this weekend I donated some quick and easy (A2-size) cards to be sent to one of OWH's shippers. See their website for complete list of rules, packing slips, etc= http://www.operationwritehome.org
The main thing to remember when making cards for the military to sign and send back home to the States is NOT TO USE GLITTER. Even small flecks can be reflective and we do not want to put them in danger.
Also for the inside layouts of cards, OWH prefers that there is limited design in order for them to have space to write their loved ones. Include liner papers, especially if cardstock/card base is dark, so that pencil or ink will show.
Also remember to include a check of donation to cover mailing costs to get cards shipped overseas. The military can send items home to US free, so the least we can do is provide them with creative cards to keep communication going to their family and loved ones.
I've also donated hand-made cards to VA hospitals here in the US to be distributed to their patients. And for the past 2 years on September 11th, I mail military-theme thank you post
cards to my relatives and former high school classmates that have had military service and/or community service (like mayor, fire depts, paramedic).