Here's a favorite:

"It’s not a method; it’s a thought. When you wake up in the morning, you have to decide how you want to see things. The mental strength consists in choosing the positive thought. That it is usually not the easiest way. It’s easier to find an excuse or thinking that there is nothing more to do."

-- Dani Pedrosa

Edit: Another:

"The real man smiles in trouble, gathers strength from distress, and grows brave by reflection."

-- Thomas Paine