Anne A. Wilson
Running or walking always seems to help. Maybe it's the increased blood flow to the head, I'm not sure, but my brain becomes "un-muddied" after exercise.
I usually listen to music when I'm working out, so if I'm mulling over a scene that I know is going to be sad, I listen to songs that would evoke that mood. If I'm working on an action scene, I go for a fast, upbeat score.
Between the music and the exercise, I can usually come up with something to push me over that hump.
If those don't work, reading always helps. No matter what I'm reading--fiction, non-fiction, fantasy, thriller--I always seem to come away with ideas when reading someone else. The bonus here is that by reading, I'm also learning. You always hear that to be a good writer, you need to be a good reader. I absolutely believe that's true. I learn something every time I open a book.
If exercising or listening to music or reading a book doesn't work to break through the block, the worst thing I've found is trying to sit at the computer and force it. It just never works and results in a lot of frustration. Better to just step away and come back later.