This is pretty self explanatory, the title says it all. (I won't post mine, because I don't have one....)
I like this quote from the Hogfather a lot so it think this should be my motto. It's the closest to a statement that covers everything, though I think having one motto is unreasonable. "Things just happen, what the hell."-Didactylos.
Sometimes it's: "Quit because you can't win." Other times: "Don't quit or else surely you won't win."