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.
Prepare for It. Your best option in this scenario is to accept it with dignity and make sure you're ready to leave the office for good when you turn in your resignation letter, whether that means gathering up your personal items or making sure you've collected the names and contact information for the teammates and clients you'd like to stay in touch with.