There's a site called OneWord that offers a very simple writing challenge. (I found it while visiting the AiT/Planetlar site, and seeing Larry's take on "cult.") You are presented with a word, seemingly plucked out of the ether, and given 60 seconds to write something about that word. Whatever comes to mind.
As writers, we see it as our mandate to think about what we write. Hell, I know I've spent years mentalizing my way through stories, waiting for them to click in a way that makes sense. But this little exercise doesn't give you time to think, or even to revise. This is the very definition of writing from the gut.
You're on the clock. One minute for one word. There's nothing to it but to do it.