Few of my favourites:
"Make it up. Make it happen."
"If you don't pay appropriate attention to what has your attention, it will take more of your attention than it deserves."
"You don't actually do a project; you can only do action steps related to it."
This is actually quote David quoted: "Vision without action is a dream. Action without vision is simply passing the time. Action with Vision is making a positive difference."
Total GTD newbie, better with the GTD theory than implementation.