1) Focus on what you like about the other person, what it is about them that makes them different, makes you want to write about them.
2) Keep it simple (again, advice you’ve heard your whole life about writing, but the more you can stick to the point, eg, your feelings for someone, the better. This can naturally flow from good editing.)
3) Edit. Some might say this takes the spontaneity out of things, but a lot of times it just takes out the bad grammar.
4) Look at some good examples to get you started.
5) Practice makes perfect.
More detailed advice:
http://www.writeexpress.com/How-to-write-a-love-letter.html
Anonymous Love Letters 