Now, I'm not saying you should 'talk dirty' in your letter, but you may want to include a little bit of sexy teasing. In most cases that will really get him going.
You know your guy better than anyone else so you have to determine the way you want to let him know you love him. Again, you have to remember that the letter is to let him know how you feel, but it should be written in a way that will make it more personal for him.
If he likes poetry, for example, try to find a line or two from a poem he likes. If he likes some other form of literature include some passage or something from his favorite book. That will allow you to express your love for him in more than just words.
When you add things like that you are letting him know that you pay attention to what he says and what he likes. He'll see that you really do love him because you're willing to take some time and make everything special, even your love letters.
If you really wanted to get fancy, you could even record a love letter and / or make a short video 'love letter'. What you put in the video is up to you (however if you're thinking of posing nude you may want to reconsider. You wouldn't be the first girl who had a 'wonderful' man and later found out that he showed your private video to all his friends).
Everyone likes to feel loved and special. If you want to show him just how much he means to you and you want to know what love letters to write to him just remember two basic ingredients: your love and his interests. Combining those things will allow you to show your guy just how special he really is.