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.
Tempting You to Stay. It's a reaction that's both good and bad: Your boss values you so much that he or she offers you a raise or promotion to stay. But it can leave you feeling conflicted: You have another great job lined up__but now that there's more money on the table, do you really want to leave your current position?