1. Send an update letter to your top choice.
This letter should state that the school is your top choice and that you would attend if accepted.
(For more on how to write a letter of intent, including examples, see The Medical School Admissions Guide)
2. Write update letters.
Send basically the same letter as your letter of intent (sans the top choice bit) to other schools you would attend if accepted.
3. Ask your pre-med advisor or a recommender to call your top choice on your behalf.
4. If you can't get anyone to call, consider sending in another recommendation (to be additional to the recs you have already sent via AMCAS or TMDSAS).
Though I highly recommend these tactics, be careful not to go overboard. I cannot tell you how many pre-meds have innundated admissions committees with updates, recommendations, and phone calls only to be rejected based on being "annoying." I suggest sending each school one communication (letter) from you and one communication (call or letter) from a pre-med advisor or additional recommender. And be sure to check with each school before you contact them. Some schools have no contact policies when you are on the waitlist. Contacting a no contact school is a sure fire way to quickly land on the rejected list.
Remember, being waitlisted means you are still in the game. Keep your head up!
Suzanne M. Miller, MD, FACEP
CEO of MDadmit: Medical Admissions Consulting and Essay Editing
How to be Pre-Med
The Medical School Admissions Guide
How To Get Into Medical School with a Low GPA