After I've written a script for Gum Girl, but before I draw the layouts, I scribble out thumbnails.
These are very quick workings out of what each page might look like. I see if there's too much going on in a page, or too little, if I need to push panels onto another page, or if I need to work a story point around a page turn. They don't look like much they're an important part of the process. It's much better to recognize and solve problems at this stage than after you've finished drawing the story.