Hmmm, that's a difficult one. I have several on my LJ - ones that mean something to me. I'm going to go for two a short one and a longer one:
"Do unto others as you would have them do unto you."
Which is the common English phrasing of Matthew 7:12 and Luke 6:31. I'm not religious, so that isn't why I like the quote. It just seems common sense to me to treat people in the way you would like to be treated. Something I fear too many people forget.
"Achievement is the knowledge that you have worked hard and done the best that is in you. Success is being praised by others, and that's nice, too, but not as important or satisfying. Always aim for achievement and forget about success."
This is a quote by Helen Hayes that I found on a card. I bought the card not to actually send to anyone, but because I thought it said so much in so few words. Words that should be headed - and again are often forgotten.
After all, there aren't many people who don't like to be praised for something they've done. But at the end of the day it really is the 'have I done the best that I can do'? which matters.