Write some random dialogue. Always works for me. Start by talking to your MC, or even a minor character:
"Hey, what's up?"
"..."
"Damn it, talk to me!"
"..."
Except when they don't want to talk.
Also, a long walk, alone, don't bring distractions like an iPod or a dog or a friend.
Also, read the last thing ALOUD which you wrote and see if it carries you into the next thing.
Also, read something else, not written by you. Get out of your genre. Read some badly-written wikipedia articles. (That ought to jump-start anybody, though I realize many articles are well-written.)
I read short stories when I'm stuck, which isn't often. They're short. They don't take long and usually I come away with, I can do as good (or better) than this, so why am I not writing?