1. Look up the celebrity's official web page on the Internet.
2. Go to your local library and look for the 'Star Directory'. .
3. Check the person's Internet Movie Database page . 
4. Search the Internet for autograph collecting sites and forums.
5. Write a nice, short letter. 
6. Include a SASE.
7. Consider including something to sign.
8. Use the right envelope and adequate postage.
9. Be patient.
10. Keep your head high.

via wikihow