Be courteous and appreciative. You may want to work for them sometime in the future.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I appreciate your offer of the position of <job title> with your company. Unfortunately, I've decided to pursue another opportunity and will not be able to join <name of company>.
I thoroughly enjoyed meeting everyone there and appreciate the time you took to review my credentials and interview me. Thank you and best of luck in filling the position.
encl: <List of enclosed items goes here>