No need to limit yourself to grocery bags (though that is what this tutorial from Terri Mauro at About.com uses). Any sort of large paper item you have lying around will do the trick. Maps, old posters and unused wallpaper would all look marvelous. A clean slate for big ideas! It couldn't be simpler.
Images: 1: Domino; 2: Happy Mundane 3: Terri Mauro