How'bout this, replacing brackets as needed:
[Date]
[School Name and Address]
Dear [Name]:
I would like to thank you for your interview offer at [School Name]. After carefully reviewing your school, however, it is with deep regret that I must inform you that I have determined that it will not be possible to accept a place in your 2002 entering class.
I evaluate comparatively every school's credentials, including your students' average MCAT scores and GPA, curriculum, faculty/student ratio, reputation by residency directors, NIH research grants, national board scores, match list, and tuition fees.
As you are well aware, I have received far more interviews and acceptances than I can possibly accept. This decision should not be construed as an indication that I do not consider you qualified to teach medicine. It is the unfortunate result of the strong competition among the many medical schools such as yourself.
Thank you for your interest in my application. I wish you well and hope you will be successful in obtaining acceptances from many qualified applicants.
Sincerely,
[Your Name]
If you want to go through the trouble, photocopy your signature on the page.