My personal favorite is from Theodore Geisel (Dr. Seuss). "Be who you are and say what you feel. Because those who mind don't matter. And those who matter don't mind."
I would also point you out to one of my favorite Seuss books, "Oh, the Places You'll Go." I won't post the full text here, because it is too long for a thread post, but I'll give you a link.
http://denuccio.net/ohplaces.html
It's not just for kids, it's very, very meaningful for adults too.