I tend to plow ahead a bit. If the first Act is stalled, but I can't stop thinking about that awesome car chase in the middle of Act 2, I will just write the car chase scene and Save it.

When I get to Act 2 leading to the chase, I do a cut/paste job and then tailor it to fit in.

If I am stalled on all 3 Acts? I whip open a fresh FADE IN and start some other idea I have and work on that one for a bit.

The end result being, always writing.