"When I was interviewing for the job with you, I was also interviewing with other companies. Your offer came in first and the job sounded interesting and the pay was good so I accepted. However, since then one of the other companies I interviewed with has given me an offer for a more interesting/better paying/shorter commute/etc. job and so I've decided to accept that offer. I hope this doesn't negatively impact you and that you can quickly find someone else."
Or something like that. Tells them the honest reason why, doesn't burn your bridge too badly.
Comments
Honesty is the best policy here:
"When I was interviewing for the job with you, I was also interviewing with other companies. Your offer came in first and the job sounded interesting and the pay was good so I accepted. However, since then one of the other companies I interviewed with has given me an offer for a more interesting/better paying/shorter commute/etc. job and so I've decided to accept that offer. I hope this doesn't negatively impact you and that you can quickly find someone else."
Or something like that. Tells them the honest reason why, doesn't burn your bridge too badly.