You've succeeded! After years earning a high GPA, getting research experience, developing relationships with faculty, studying for the GRE, wrestling with grad school admissions essays, getting reference letters, and interviewing for grad school - you get word that you're in. You've been accepted to graduate school!
Thanks, but No Thanks
After carefully considering your options, you're ready to decline and offer of admission. How do you do it? Not by a long-hand letter. A short, simple email will do. Try something along the lines of the following.
Dear Dr Smith (or Admissions Committee):
I am writing in response to your offer of admission to the Clinical Psychology program at Graduate University. I appreciate your interest in me, but I regret to inform you that I will not be accepting your offer of admission. Thank you for your time and consideration.
Rebecca R. Student
Remember to be polite. Academia is a very small world. You likely will encounter faculty and students from that program sometime during your career. If your message declining the offer of admission is rude, you may be remembered for the wrong reasons.