Dear (Name of your boss):

This is to inform you that I have accepted a position with (Name of your new employer).

I want to assure you of my gratitude for a rewarding professional association during my employment with (name of your current employer).

This decision was not an easy one, and involved many days and hours of thoughtful consideration, particularly with respect to my own plans for the future. Nevertheless, my decision is final.
Please do not make this process more difficult than it need be by discussions of the reasons for my decision, or whether or not it can be changed.

My main thoughts are now to work as hard as possible to wrap up my affairs here, and turn over my responsibilities as you direct. However, I would like to join (name of new employer) as soon as possible. Therefore, I request that you waive my termination notice, if convenient.

With sincere thanks and best wishes for the future,

(your name)