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A long while back I mentioned that writing a synopsis will help focus a novel.  It will do nothing but good, even if you abandon said synopsis in the course of revising the novel.  

Another good reason to write a synopsis early on in your project is that writing a synopsis is hard.  A good synopsis will take a long time to write unless you get lucky or you think in terms of synopses and can write a 'take me, take me now, take me hard!' synopsis in an hour or less, in which case I hate you and you don't need to come back here ever again.    The synopsis has to go through the same review process as any other writing you plan to submit, so waiting until you're all done with writing your work will only delay your submission.  You have to write it, get it critiqued, rewrite it, get it past your best readers, give it a final polish, and don't forget to kiss it goodbye when you send it off, or at least give it a good spank on its tight little butt on its way out.

Here's some help I found recently thanks to a link of a link of a link.