My number one tip for writing letters is don't force it. If you sit yourself down and try to force yourself to write a letter, it will come out forced. Wait until you're feeling especially sad or are especially missing him. You know that overwhelming depressing feeling that bubbles up sometimes or that moment that sneaks in that you just miss him so much you have to lie down and have a good cry? That's the perfect time to write your letter. Release that extra emotion by writing a heartfelt message to him. It doesn't matter how mushy or cliche the words sound. I had a lot of those emotional moments right before he left and at the start of the deployment so I had quite the stack of letters ready to go.
Another idea of what to write: your plans for when he gets back. But this might be a personal preference of my husband's. He loves planning our future trips to distract himself from his current location. He also loves talking and hearing about how we'll have days of cuddling on the couch watching all the movies he missed while he was gone and the tv shows we have to catch up on. However, I know some guys hate talking about that kind of stuff because it makes the homesickness worse for them.
Sometimes I simply take a selfie with my iphone, print it out, and send it with a simple "I love and miss you" written on it.
One of the pictures I printed and sent...nothing special, just an everyday me.