Keep your resignation letter short, simple, and positive. You may want to write something such as:

"I want to thank you for all you have done for me here at (Company). It's been a pleasure working with you, and representing the company as your (job title).

I have accepted an offer with another firm and have decided to tender my resignation. My last date of employment will be (approximately 2 week notice). This decision has nothing to do with the exceptional opportunity you have provided me here. You and the company have been more than fair with me, and I genuinely appreciate all your support.

I wish (Company) continued success, and I want to thank you for allowing me to be a part of your team. Please feel free to contact me at any time if I can be of further assistance in helping with a smooth transition."