Gentlemen:

Effective today, please accept my resignation. I have accepted a new position offering me a most advantageous career opportunity. It is a position in which I believe I can excel, and my decision to make this change is irrevocable.

I would like to take this opportunity to thank you for all you have done for me during my tenure here. I very much appreciate the invaluable experience I have gained through my association with you.

Sincerely,

(your name)