I draw pictures of my characters, settings, and objects relating to my story. Sometimes this can help you come up with new ideas. Also, work on the backstorys of your protagonists, antagonists, and world. This can be especially helpful if you're book is set in a made-up place, like Middle Earth. You can also work on the family trees of your characters, and find out who their parents, grandparents, uncles, aunts, and cousins are. This can be very helpful when coming up with plot twists.