Last week I received a rejection slip by email from a publishing company I don't even remember submitting to. What are they doing, passing my sample chapters around among themselves now?
When I worked in teaching, I had a colleague who pinned to his office noticeboard all the complaint letters he'd got from parents. (He had a lot.) Some people do the same with rejection letters. It's always a balance between the poles of Lord of the Flies (seventy-odd rejections before publication) and 'There's a Reason for Rejection - Your Book is Rubbish, All Your Friends Lied To You So They Wouldn't Have to See Your Crushed, Defeated Face.'
I did get some excellent help from an agent who read my draft at its monster-in-a-shoebox stage. She told me to cut it in half, so I thought, kill your darlings, be merciless, and excised 100,000 words out of it to make it leaner, tighter and frankly much better. Then she rejected it.
I take criticism like an egg takes a housebrick, so I try to cultivate briskness. So the preferred method is a quick "bastards!", ping the letter into the fire, cross another one off The List of Bastards, and carry on.