One of the most important lessons that I have learned through bitter experience is "Don't forget your spine". This does not mean that I'm wimpy; in fact I have been told that I can defend my positions with gusto; no, I'm talking about the spine of your story - that one sentence that sums everything up. The 'what happens when' or 'what happens if' or 'Why...' that got the ball rolling back when the hints of the plot first starting knocking on your consciousness.
Yesterday I was talking to someone who is having trouble writing a play. They have done mountains of research and adore the subject matter. I asked "What's your play about?" and answer came there none. I expounded my theory that without the spine no one will get anywhere. The spine becomes your roadmap, your litmus test. I use it constantly to assure myself that I'm staying on track. Does each sentence push the plot forward, add a new dimension to a character, color a scene? If the answer is yes - the sentence (or paragraph) stays. If not - out comes the highlighted delete weapon of destruction.
The spine, ladies and gentleman. No book, play, screenplay etc. can survive without it.