For my Dark and Fevered Dreams book series, I'm using more and more illustration. When Volume 3 hits (probably later this year), it will be much closer to a graphic novel than any of the previous editions.
As a result, I'm spending a lot of time planning visuals so they enhance the written word. I found this article with illustrator Mimi Chao very helpful as she talks about the art of storyboarding and some of the valuable tool she uses. Aside from the obvious benefit to filmmakers, this might be a great resource for other artists/authors, too.