I sent a short letter saying something like "Dear _______ Admissions, After careful consideration I have decided to withdraw my application from X SOM (or "withdraw from the XSOM class of 2011" if I was accepted and want my deposit back). Thank you for your consideration." Short and sweet. I don't think most places will accept phone calls. And I thought a snail mail letter was more formal, though I'm sure email is fine, too.
I assume that means you got into multiple schools - congrats!!