I've been organizing a lot of bookcases lately, mostly in playrooms it seems, and I have two simple and easy steps to share:
1. If your bookcase is full of books, sort by color. I've found that my girls actually put books back better when they are sorted in a rainbow instead of by series. After all, they can't read, but they know colors- plus it's pleasing to the eye!
2. If you have room for other objects on the shelves, start by sorting books by size first, and then color. Place similar sized books on the same shelf and large books flat on their backs.
3. Bonus tip! I place all paperbacks in either a baskets or cubed bin. Typically paperbacks make the bookcase look a little too "junky".
Hope these few tips help!