Dear Mr./Ms. XXX
Thank you for the time and effort you spent considering me the final interview. I appreciate your time and effort -- as well as those of your staff. I am grateful for your meeting that I attended before .
Because I was so impressed with (another office that you've selected, I had a difficult decision to make. After much thought and careful deliberation, however, I regret to say that I have decided to reject your interview invitation.
I wish you and XXX (the name of this company). the best continued success. I hope our paths will cross again in the future.
Thank you again for your time and consideration.
Sincerely,
(Your full name)