Read this. It's Jim Butcher (The Dresden Files) writing on his strategies for making it through the dreaded middle of your manuscript. I've intuitively used a couple of the strategies that he mentions, as you probably also have, but I wasn't at all organized about it. Having such concrete thoughts and methods as he presents is likely to be a really big help to me, and I hope also to you.
Many thanks to Dwight's Writing Manifesto for mentioning this post in passing (that's another pretty cool blog on writing you should check out, by the way).