And sure, this could be an email (all of the same principles would apply), but a letter makes things official in every sense. It also gives you a prop to take along to the face_to_face meeting with your boss, when you let him or her know that you're leaving.
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.