The two things that work for me are:
a) Just keep writing, one word at a time, even if it's so boring I could scream - until I find the story moving again (easier said than done, I know). Then I go back to delete what didn't fit if necessary.
b) I sit and write stream-of-consciousness style about the story. I'll take a notebook and brainstorm about the story, pointing out the holes and throwing out ideas on how to fix them until I find one I like enough to run with it. For last year's completely goofy story, I have pages of things like "Why would the Toaster of Doom use llamas to do his dirty work? What is it about llamas that makes them ideal henchmen? " Etc.
This year I'm writing slightly more serious stuff, but the brainstorming works about the same...
The second thing I do is I write dialogue. For me, dialogue takes on a life of its own. I write things that I never thought about before, and dialogue is easier to write than description.
I have to ask...did the Toaster of Doom use llamas to do his dirty work? What kind of dirty work would llamas do?
Did you ever see the Calvin and Hobbes where Calvin discovers the toaster. He puts the bread it, pushes the lever, the toast pops out. Hobbes says that its a neat trick. Calvin looks in the slot "yeah, but where did the bread go?"
Well for one thing, they spit.
Evenin'.