Holly Lisle has a short video of how she gets unstuck when she’s writing (movie starts when you click on the link). I think this is a good method of letting your subconscious loose a bit (I wanted to take some pictures she discarded …)
Usually what I do is to disengage from the book and do something else. Work on a different story, go outside, take a shower, etc. (not all at once) I’m a big fan of water. Swimming, washing dishes, watering the grass. This is how I get my best ideas.
The key is to find what works for you. If you’ve found a good method, post it here. Maybe your idea will help someone else who hasn’t quite found it yet.