State Your Intent. The whole theme of your resignation letter is the statement that, well, you resign. You don't need to play coy__the time has passed for you to fish around for a counteroffer if it's gotten this far. At this point, you've made your decision to leave, and this letter needs to reflect that.
Acceptance With Grace. The best_case scenario__and honestly, the most common reaction__is that your boss will accept your resignation with understanding and sincere congratulations. Your manager will be happy to see that you're advancing your career and moving on to something bigger and better.