I've been sending letters every day. I remember the joy of a mail call in which I received something, and the dull ache when I left empty handed.
Letters don't have to be anything super detailed or inspirational, just letting your cadet/mid know what's going on around town, stupid things the dog (or their sibling) may have done, and that you're thinking of them and proud of them.
I always end the letter with a joke or humor posting, usually about the military (the best target of the joke being one of the other services...it humors the cadre). I also try to include the last few days of his favorite comic strip too. There is a good web site for cut and paste of the strips, too. It's rather creatively called: www. comics.com. It has almost all of the strips you'd expect, and then some.
Still haven't heard anything back, but as I said earlier, I almost don't expect to, except maybe the ice cream social call on July 18th (USMA cadets). It's hard on my wife, but again, we both know our kid and know what to expect.