Some suggestions are checking with your local veterans organizations (such VFW, American Legion, Marine Corps League) or any service organization (Lions Club, Rotary, Kiwanis, Moose Club, DAR, etc) to see if they are currently running any coupon collecting projects where you can physically drop off the coupons to them.
If you cannot find a project in your area, then you can talk to your friends and neighbors, co-workers, to see if they know of any military families stationed overseas who might want to be on the receiving end of coupons and you can mail directly to them.
Remember that it costs the same amount of money to send coupons directly overseas as it does to send to a local group, so if you're going to be mailing, you might as well just mail direct, otherwise the group doing the collecting is paying postage for a 2nd time for the same batch of coupons.
Not only is there information on how someone can get started sending coupons as an individual or as a group, but also a project directory of different groups around the country, where you can physically drop off coupons and the groups will mail them out.