Prepare for It. The best way to prepare for this reaction is to simply know your stance ahead of time by determining if you would even consider a counteroffer. Yes, it may be more money or a better title, but there's more to consider__including your career goals, level of job satisfaction in your current role, and the opportunities that your new role offers.
After that, you're ready to close out your letter and move on. The basic closings are fine here__again, you don't need to go overly sentimental or formal, but you should also write as if it will be read by anyone at the company. And with that signing flourish, you're done! You've resigned, and you're ready for your exciting new job.