personally i prefer planning the novel before i write - it helps when you get stuck somewhere, especially if you have some sort of an outline. and it's fun.
then, after planning and everything else in between, Just. Keep. Writing. just keep on it until your fingers fall off - you know, in between real life and stuff. don't let anyone, not even yourself, tell you that the book you're writing is crap and that you'll never finish it. before you know it, you're typing out "the end" at the bottom.
this might help too:
http://jimbutcher.livejournal.com/ - i love jim's lj and his thoughts on writing and stuff - it helps me lots. lol.
ALTHOUGH, that's for planners like me. if you're more of a jump-straight-into-the-fray type of writer...
just start writing and don't think much about it. just see where your characters, story, and fingers will take you - let it flow and all the metaphors that come with it. take the idea and run with it, and before you know it, you'll be finished.
and if ever you need a driving force, a deadline, a challenge to get you *started* have friends place a bet and give yourself a deadline, or, you could try NaNoWriMo.
That's what works for me, at least.
right now i'm stuck with the problems of self-editing. haha.