Dear Personnel Manager,

It is with great sadness that I am hereby tendering my resignation to you.
While there is much to say, I believe you know the reasons leading to this decision, and I will therefore leave them unsaid at this time. If you wish to discuss the reasons, I would be willing to meet you in private.
I appreciate having had the opportunity of being a member of the __________________ Corporation for so many years and offer my best wishes for your continued success.

Sincerely,