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.
Prepare for It. In this case, all you can do is politely deliver your resignation, then strive to make the transition as easy as possible for your team. Your manager may not have the ideal reaction to your news__and your workplace may not be the most pleasant environment for the next two weeks__but to avoid burning bridges, it's best to work through it with a positive attitude.