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. You may receive this reaction if you're a high_ranking executive or director. These positions can be difficult to replace quickly, so it may be worth it to your company to tempt you to stay rather than letting you go without a fight. It can also happen if you have extremely specialized knowledge that would make it challenging to replace you. On the other hand, if you're in an entry_ or mid_level position, and someone could be hired into your position without much trouble, it's less likely your boss will put up a fight__and a counteroffer.