[Date]
Dear [Celebrity's Name],
I have been a fan of yours for years, and have enjoyed your many [movies, songs, etc.] My favorite is [list your favorite work here]. I was wondering if you would mind sending me your autograph. I have enclosed a self-addressed stamped envelope for your convenience.
I hope to hear from you soon, and deeply appreciate your generosity in fulfilling this request.
Thank you.
Sincerely,
[Your Signature]
[Your Name]
[Your Mailing Address]