My very personal favourite is -
"Heights by great men reached and kept, were not obtained by sudden flight;
They, while their companions slept, were toiling upwards in the night! - Henry Wadsworth Longfellow
Some others that keep coming back to mind and that one tries to adhere to are -
"Live such a life that when you die even the undertaker would be sorry," - Mark Twain
"If you don't stand for something, then you will fall for anything," - Malcolm X
"If you tell the truth, you don't have to remember anything."
"Do unto others what you'd like them to do to you," - I wish I could follow this one religiously.
And since I am a self-proclaimed Dog Whisperer, here is an absolute must -
"Dear God, please make me the kind of person my Dog thinks I am."