How do you jumpstart a scene?

In regard to plot stagnation, Raymond Chandler famously said, "When in doubt, have a man come through a door with a gun in his hand."

TV Tropes has a lot of fun discussing what they've dubbed "Chandler's Law," and comparing it to parallel plot devices in TV, comics, films and video games.  I've heard that NaNoWriMo participants often run out of steam during the second week, and have to rely on quick, sometimes outrageous plot devises to keep their "writer's high" (and word count) going. The only time I attempted NaNo, I didn't even make it through the first week, so I'm not exactly an expert.

When your story doesn't seem to want to move forward, how do you jumpstart it? Have you ever used the same fix twice? Did it work?