The late Jim Rohn was one of the greats in the field of personal development. The author of several books, Rohn was also a mentor to Anthony Robbins early in Robbins's career.
Here are a few quotes from Jim Rohn:
"Affirmation without discipline is the beginning of delusion."
"Better understated than overstated. Let people be surprised that it was more than you promised and easier than you said."
"Don't bring your need to the marketplace, bring your skill. If you don't feel well, tell your doctor, but not the marketplace. If you need money, go to the bank, but not the marketplace."
"Don't wish it were easier, wish you were better."
And a video: