I know it sounds trite, but you gotta leave your emotions at the door.
You will get outdrawn, outplayed and very badly beaten by the worst of (or maybe it is the best of) the donkey pokertards.
When some nitwit is smiled upon by the pokergods and all of your beautiful play for the last 4 hours is crushed to nothing by the player who does not have the good sense or good manners to realize that he/she is not a poker genius when they happen to get lucky on the river, your best play is to shrug and smile.
I still get steamed at times, but since I have been working on getting unemotional, I do find that I go deeper in more tourneys.