I actually have a good one to share. It started out like some of the others - somebody had been mean to me somehow (late junior high or early high school)...but that's not the part I remember. What I remember was my dad's reply:
"Be patient with them. They'll grow up eventually."
He went on to explain that they were most likely acting out of immaturity, not malice; that I was just past that phase and they hadn't caught up yet. - Who knows, maybe he was just trying to make me feel all mature and special...but it turned out he was right. My class did eventually grew out of "cliquiness" and gel really well, and when that happened, I was glad I hadn't held onto any resentment. I would have missed a lot of fun my senior year if I had!
I've extrapolated that answer to a lot of situations in my life, and more often than not, it keeps me from interpreting bad behavior as personal. It doesn't mean I tolerate being treated badly, or won't remove myself from an unhealthy situation - but it does go a long way toward not having it eat away at me. It's very freeing to assume people are works in progress, and leave it to them to figure out their stuff rather than me to react to it.