Whenever I get stuck on a piece, I leave it for a while. As I like to call it, I've run out of 'creative fuel'. I might leave it for twenty minutes, maybe a day, but once the 'creative fuel' is filled up again it hits me unexpectedly. A lot of the time I'll be browsing the Internet and be playing random stuff, and that's when I'll get the, "Oh, that sounded pretty cool!".
It's a matter of time and a clear, relaxed mind.
But much like what others said: look at what you have so far, pick out the best parts and base go from there.