The time I have spent at (company) has been most rewarding and helpful in my career, and I hope that my contributions to the company have been constructive.  My relationship with you has always been professional, warm and results oriented.

I have accepted a position that will enhance my career growth and will expose me to challenges and opportunities which I believe are in my best interest and my families best interest.  Should you flatter me with an offer to remain, I could not, under any circumstances, consider it.  My decision has been made it is irrevocable and final.

(Boss’ first name), I have the utmost respect for you and wish you nothing but the best for you and (company).  If I can be of any special assistance during my final two weeks, please feel free to call on me.

Sincerely,