Just read two great articles from author Hilari Bell. Check them out:
Twisted Plots: the necessary change of direction A snippet: "... a good plot twist isn’t just a story event that the reader didn’t see coming, or something that surprises the protagonist. In order to change the story’s direction, a plot twist must be something that forces your protagonist to change his plan."
Don’t Skin the Cat: better ways to solve a story problem Another snippet: "...sooner or later you’ll be told that something in your story isn’t working and you should probably cut it. But should you? The answer to that is a solid, definite, maybe. Sometimes you can follow Tim Gunn’s famous advice and Make it work!"