I like this line from Ray Bradbury's speech at Caltech:
"Now, right after graduation today, make a list of the people who don't believe in you. And you have a few, don't you? I had plenty of people who told me not to do what I was going to do. You make a list this afternoon, of the people who don't believe in you, and you call them tonight, and tell them to go to hell!"
http://pr.caltech.edu/commencement/00/b ... peech.html
The best advice I've heard was Ali G's commencement speech at Harvard:
"U has all got de potential to become great americans. And remember America is de greatest country in de whole world ...apart from Jamaica...and Holland.. oh yeah and Thailand coz u got dose girls who do all de ye know and probly some others - but u iz definitely in de top 20. U people iz de future, u has de chance to change de world, to hactually improve de life of de poor, OR U could goto Wall Street and earn millionz - get plasma screen, chinchilla coat, a series of relationships wiv gold-digging hoes happy to de de most disgusting sexual favours for some bling. Don't waste de opportunity dat g-d has given you - see u in Wall Street."
http://www.hbo.com/alig/harvard.html