Do It and Screw It! (what's your motto?)
"Do it and screw it" is a phrase I heard from my friend, super-cartoonist Vanessa Davis. I think it comes form her mom. It just means stop worrying and get on with it! Do it!
Also there's "When in doubt, black it out." That's from my former professor and current boss James Sturm. It's just a practical recommendation when designing a panel. James would probably agree that Tom Hart's "When in doubt, use tracing paper" is more helpful. Tracing paper"s such a great tool, it can save you from ruining a drawing.
Then there's "Make every moment a working moment." That's from K Thor Jenson. Thor has been producing comics steadily since he was a teenager. He's an advocate for drawing whenever you can, like on the subway. Your sketchbook: don't leave home without it!