Step 1: Acquire new shelves.
I had been storing my books on a huge bookshelf I got from a store that was going out of business. The problem was that I frequently take sections of books out for thematic units. The remaining books would fall over, creating a mess.
My son had a huge set of the white "cube" shelves from Target. I confiscated them and took them to my classroom. He was using the shelves to store collections of Bionicles and Legos that he was pretty much done with, so it all worked.
Step 2: Make new genre markers. For years, I used tags I had made out of cardstock. They were pretty beat up. I made a new set out of a cute "circle badge" design I got for free from The Hazel Owl. I laminated them and glued them onto paint sticks that I spray painted green.
Step 2: Move the books and insert the genre markers. The books seemed to all fit into their sections!
The final result is so much neater and easier to manage!