Dear XYZ school,
Due to the great number of acceptances I have received, my school selection committee (Jack Daniels+dart board) has determined that what your school has to offer me is no longer competitive to secure me in your incoming class. I realize that I have shown considerable interest in attending your school by previously accepting a seat in your class, and this may have adverse effects on your ability to further recruit highly qualified students to your school. I liken this however, to the process of medical schools foregoing a legitimate screening process and sending both competitive and non-competitive students time consuming secondary applications and budget depleting application fees. Frankly, I never had any true intent of attending your school. I am sure you will succeed in your pursuit, as you certainly have potential, to find qaulified individuals to fill the remaining seats in your next class.

(yes, i would incorporate emoticons too)
Sincerely,
dradams