When I can't finish a part, or a song, I'll grab a tape recorder. Then I'll record myself while I sing the part that I do have finished. No instrument, just halfway mindlessly singing the melody or the riff, like when you walk around and hum a tune that's been stuck in your head all day. But instead of stopping when you get to the part that isn't finished, keep on singing. Don't think about it too much, just keep right on humming or singing. Chances are, you'll end up singing exactly what the next part should be. A lot of times your brain knows where it wants to go but we get so caught up in thinking about it that we don't just let it flow. I've fixed the "how in the hell do I finish this song?!?!?" problem this way a bunch of times.

THRASH IN OUTER SPACE TO DEATH.