A Gracious Tone. Your resignation letter is not the time to air grudges, take passive_aggressive shots, or otherwise be unpleasant. Regardless of how you feel about your (soon to be former) job or your boss, this letter should be an exercise in professionalism and graciousness. An excellent way to do that is to say a simple "thank you." Even if you hated your job, you got something from it at some point. Focus on that positive bit if you need to.
Spot the Signs. Do you have a good relationship with your manager? Is he or she relatively reasonable? Do you have regular one_on_ones where you talk about your career goals and growth? If so, it's likely that your boss will accept your resignation with grace, help develop a plan of action for your last two weeks, and maybe even attend your farewell happy hour.