You can use the format I've been using, which by-the-way, I borrowed from Polar Bear I think:
To whom it may concern,
Thank you, and the College, for your interest in my application to the University of XYZ. I sincerely appreciate your time and consideration in my review. However, I am writing to let you know that I have received an acceptance from another school, ABC, and will be attending ABC in the fall. As such, I would like to withdraw my application from the applicant pool. Thank you again for your consideration.
Kind Regards,
SDN superstar
Edited for content and made more generic, you don't that twang in your withdrawal.