Sure, it may sit in the depths of HR until the end of time, but you can be sure of two things: One, your former boss (and future reference) will read it and be impressed. And two, if you ever decide to come back to your company (hey, it happens), it's a very good thing that last thing on file is a great, professionally written resignation letter.
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.