Here are some tips from ehow.com:
1) Sit quietly for a few minutes. Picture him in your mind. Think about the way he makes you feel and why.
2) Brainstorm some ideas. On a scrap of paper, jot down a few key words based on your thoughts about him. Is he strong? Is he dear? Is he funny? What about him makes you love him? What attracted you to him? Make notes about specific experiences you have shared. Also, jot down some action words to capture his attention. For example: kissing, cuddling, stroking. You get the idea.
3) Write your sentences. Choose your favorite thoughts from your brainstorm and formulate them into sentences. Maybe start with what attracted you to him, and be specific. Instead of saying, “I love your sense of humor,” say, “I loved it when you copied that waiter’s accent and he caught you, and then for the rest of dinner you pretended like you always talk that way. It was the funniest thing I’ve ever experienced!”