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.
However, if you know the right signs to look for, there's a good chance you can figure out what kind of reaction to expect__and that can make the conversation go much more smoothly. Here are a few common reactions and how to anticipate which one you'll receive.