After much trial and error (stress on the much), I have found that the best way for me to nail down any story I'm working on is to come up with one sentence that describe the over-arching idea of my book/play/movie/whatever.
Yes, I strongly suggest figuring this logline (as screenwriters and other movie peeps call it) before you sart writing your first draft (of course, to help you do this you'll probably have concocted a bunch of ideas or done some research already).
Why do I feel that way? Because it forces my over-imaginative (active?) mind to FOCUS (or, as the French would say, Fohk-Us. Sorry, inside joke. It's... never mind). This is definitely a crucial word for me as I have found that without this backbone, I have a tendency to wonder all over the place. Which makes for very boring, unnecessary (doesn't one imply the other?) @$#% that I later need to cut.
Pile of my previous drafts (not including the current one nor, yes I know, the one that's probably coming after).
Ah, if only I'd known this at the very beginning, I'd have been able to skip drafts 1-6, at least (yes, currently working ondraft 14--hey, I'm learning!).
Note: yes, I do recycle.
And no, I don't feel having this one-liner at the beginning is in any way constraining. I mean, it keeps me focused on the topic I want to explore, yes, but that's it. Apart from that, I can let my imagination run wild, and often times it actually helps me be even MORE creative as I find other ways to string my scenes together.
Finally, it's not because I already have a logline (or a plotline) of what I want to happen that it can't change here and there (though making changes can be quite complicated as then you have to go back all over the place to make sure it all holds together still).
Here's a great post if you want to read more about the One Sentence Stress Test.
--The Writing Apprentice