Like Julia, I almost always sketch my designs. They rarely turn out exactly like the sketch, but improvising everything is usually disastrous for me. For more complicated designs, I even make lists of the colors, consistencies, and order of steps, especially if I'm multitasking two or more designs in one session (but I'm a crazy Virgo). Most of my sets are 3-4 designs only. I don't think I've ever made a set with more than 8 designs. Even as prepared as I am, I still procrastinate a moment before committing icing to cookie.