I occasionally use post-it notes to jot down very rough thumbnails. They make a size that has nearly the exact same proportions as a book cover. I then either stick them above my computer, hand them off to a designer or throw them out. Looking through some of these from the last couple years made me realize how few of them actually make it through to final. Part of it must be the book cover kill rate, but I also think I might be using this process as a way to get the bad ideas out of the way, though for the life of me I have no idea what projects half of these are for.