I was only on the alternate list at SLU but I have absolutely no desire to go there so I pulled out. But this is what I wrote them, maybe you'll find it helpful:
Month Day, 2004 First Name MI. Last Name
My Address
My City, My State 11111
My AMCAS ID
Saint Louis University
School of Medicine
Admissions Committee
1402 South Grand Boulevard
Saint Louis, Missouri 63104
Greetings,
I would like to thank the Admissions Committee at the Saint Louis University School of Medicine for its consideration of my application for admission. I will be pursuing a medical education at another institution. Please remove my name from the alternate list and withdraw my application from further consideration for admission.
Best Wishes,
My Name