By watching the movie that plays out inside my head and writing it down.
To me, a good novel is something that you can visualise. You need to be involved with it on a personal level. You need to see the characters; to see the scene. If you can't, then you'll never identify with them - you'll never understand them.
If I'm not doing that, then I feel like I'm failing. So, I guess, motivation is my inspiration. I don't do this for anyone but myself.