Below: a brief letter of advice he wrote to an aspiring artist and fan of Haring's work, circa-1987.
Transcript follows. Image courtesy of Gotta Have It.

Image: Gotta Have It
Transcript
KEITH HARING
676 BROADWAY N.Y.C. 10012 212-477-1579
Michael -
Thanks for your letter. I draw everyday. When I was 15, I wanted to be an artist so I drew all the time. It was my only visible talent.
Whatever you do, the only secret is to believe in it and satisfy yourself. Don't do it for anyone else.
Good luck,
Keith