Some of my favorites:
"Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great." Mark Twain
Nothing beats a failure but a try!
Noone can make you feel inferior without your consent - Elenor Roosevelt
And the one that is in my signature line:
What I do today is important because I am paying a day of my life for it. What I accomplish must be worthwhile because the price is high.