you should go something like this
Dear Princeton University,
Thank you for rejecting my application for the fall 2012 admission. While I assure you that your rejection letter was carefully reviewed by me, I regret to inform you that I am unable to accept your refusal to offer me admission to your department.
This year, I have been particularly fortunate in receiving an unusually large number of rejection letters. With such a varied and promising field of programs rejecting me, it is impossible for me to accept all rejections.
My rejection decisions take into account not only the universities’ prestige, but also the suitability of the department’s program to my personal interest. Hence, despite your university's outstanding qualifications and previous experiences in rejecting applicants, I find that your rejection does not meet my needs at this time. Therefore, I will join the ranks of graduate students in your department this fall. I look forward to seeing you then.
Best of luck in rejecting future applicants.
Sincerely,
Meep