By nature, I'm stuck in a chair all day in front of the computer, or brainstorming on paper, but the irony is that some of my best ideas come while I'm in motion.
If I'm stuck on a scene, or struggling for new ideas, I've found the best way to open my brain is to go for a run. Other mindless activities work as well, but running is my personal favorite, and it has the added bonus of burning some calories.
I think part of the value is that while running, I can't focus too hard on anything. My mind wanders, and ideas flit in and out, but my subconscious mind is more active. I'm not “forcing it”.
The only drawback is capturing the ideas before they flit too far away. I use my handy iPhone recording feature, but I used to repeat the idea to myself all the way home, or try to spin it into something larger and harder to forget.
It works for me. What works for you?