Keys to Drawing from Imagination
I borrowed this book from the library and though I only got through the first couple of chapters before I had to return it, I liked where it was going. The first step is to "doodle with purpose." It's easy to do and it makes a great warm up exercise. Parka has a great review of this book here. Actually, if you like art-of books or instructional art books Parka is a great blog to subscribe to.
Alchemy
This is an open source drawing application that is specifically designed to engage your right-brain. It has tools to automatically mirror and even a random brush that creates unexpected shapes that can then inspire you to follow a direction you would never have thought of on your own. It's a bit hard to explain in words so check out this video of Alchemy in action if you're interested:
There are quite a few examples from users on YouTube.