The most important thing in a resignation letter is to be professional and courteous.  Do not burn a bridge.  The letter should be brief, firm, to the point, and gracious.  There is no need to elaborate, to explain, or to apologize for your decision.  Everyone resigns at some time during his or her career.  Short and to the point leaves little room for retribution, renegotiation, or counteroffers, all of which can harm you in many ways.  (See articles, including Career Suicide: Never Accept A Counteroffer).  We suggest the following: 

Dear [Recipient's name]:

Thank you for the opportunity to work at [company name].  I truly appreciate your guidance and mentoring during my employment, and I thank you for all that you have done for me.

I have recently been presented with an outstanding career growth opportunity, and have decided that the time has come for me to move on. This letter is to formally announce my resignation from [company name].  My last day will be [two or four weeks from today].

Please let me know how I can best help to make this a smooth transition for you during my remaining time here.

Regards,

[Your Signature]

Your Name, Typed