When you get a new project brief or that first wave of inspiration, thumbnails are where you begin. They are the result of a brain dump. Many terrible sketches and ideas should be drawn and not judged! Make as many thumbnails as you can dream up (time considered, of course). Share the best ones with the client for direction, hide the ones you're not happy with. Burn them quickly before anyone sees you can't draw hands perfectly every time. Just kidding. Keep them for reference later.