Why a Resignation Letter? In these digital times, why should we submit a resignation letter, anyway? A paper trail does seem old_fashioned, but in this case, it's a best practice. It makes your intentions clear, as well as the conditions. You're removing any ambiguity about the end of your time there.
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.