Thank you for the time and effort you spent considering me for a position as xxxx. I appreciate your time and effort -- as well as those of your staff. I am grateful for your offer of employment.
Because I was so impressed with xxxx (company name), I had a difficult decision to make. After much thought and careful deliberation, however, I have decided not to accept your offer at this time.
I wish you and xxxx (company name) the best continued success. I hope our paths will cross again in the future.