Uh, it's pretty simple. Something to the effect of:
"Dear ____:
I would like to thank you for the time and effort spent in reviewing my candidacy at _____ Medical School. However, at this time I have decided that I am unable to accept your offer of admission, and wish to withdraw my application. Thank you again for your time.
Sincerely,
Synite"
They're not going to spend time poring over it. The main thing is not to be an ass -- keep it short and simple.
As an aside -- when you withdraw your acceptances, most schools send out an anonymous survey asking for your reasons why, and how you would rate their school. They will also ask which school you are attending instead. This is basically for the schools to find out what they may be doing wrong in attracting students. I urge you all to be as honest as possible in this survey, please. It is a huge favor to the the school and all the students at the school for you to do this. If the lack of P/F grading bothered you, say so. If you wanted more PBL or less PBL, if you needed a better financial package, lack of time off for the boards, etc. Obviously, things like location can't be changed. But if there are aspects of the school that could be improved on, it is really, really helpful for the school to hear about it from the applicants. Sometimes it's more helfpul hearing about it from the applicants than the current students. Schools like to have high yield rates, so if there is one aspect that is really deterring students from attending that can be changed, odds are it will be changed. Consider this a nice service to all the med students who hosted you on interviews, led tours, answered Q&A sessions, etc.