I would suggest writing a letter that not only expresses your interest in that particular school, but that also gives an update of what you have been doing to strengthen your application since being waitlisted. For example: If you have been continuing to volunteer (or have started a new volunteer activity) mention it. Any new honors/awards your could mention. You might even mention new hobbies, if you can spin it to make yourself sound well-rounded. Try to make it conversational, but not braggy. Just so that you come across like this: "This is what I'm up to now....I wanted to give you an update because I am continually trying to strengthen my application.... I am really excited about your school/medicine in general....etc.." That's what I did, and I got a call from the school a week later saying I was in! (Coincidence? I think not.)
Also, call the admissions office periodically to find out about movement of the waitlist. Not every day or anything, (that could get pretty annoying) but often enough to remind them of your existence.
Just suggestions, of course, but I really think that being proactive helps. Good luck.
