Lisa recently mentioned writing in a change of scenery.
I find that a change of scenery often helps me, particularly when I'm struggling with a difficult scene or sequence. This usually involves going to Borders or Barnes & Noble, getting a mocha, and sitting in their cafe area with a yellow pad or steno book.
An hour there typically results in several pages of writing goodness.
Of course, being at Borders or Barnes can sometimes be hard on the wallet.
All those books...