Please accept this letter as my formal resignation as [your position] for [company name] to become effective as of [resign date]. I have accepted a position in [state, place].
I believe this position will offer me more challenge and opportunity for advancement as well as allow me to broaden my own skills, experience and knowledge.
I want to take this opportunity to thank you and [company name] for your time and efforts in my training and advancement during the last [time period]. The support and concern shown for me has been deeply appreciated. There will be no need for the extension for a counter offer as I would not look favorably upon it.
I wish you and the company continued success, and should you need my assistance in any way, please do not hesitate to let me know.