the email i sent them was really brief and to the point...maybe 4 or 5 lines explaining that i currently have a deposit down at another school but that i continue to be highly interested in their school and would be honored to receive an acceptance offer.
the letter i had written the month prior had been more in depth as to why i was interested in attending their school (it was about one page typed, 12 font size). i didn't make any promises such as "if accepted, i will undoubtedly attend" because i knew that was not the case.
i don't think getting off these lists is an exact science (otherwise i would have probably heard from the other 5 schools that waitlisted me too) but i think short and to the point is the way to go whether that be via email or letter. they just want to know that you are still in the ballgame.