At some point in my semi-random wanderings around the internet, I clicked on a link to Storyfix. I believe it was on Margo's blog (which you should absolutely be following, for the record)
Storyfix is Larry Brooks' blog, and one of the absolute Must of writing blogs out there. This is where, on the internet, I have the learned the most about the craft. In fact, I think that'd be where I've learned the most, period.*
I could preach his word and repeat how important story structure is. I could re-explain his concepts. Or I could, you know, redirect you all to the Story Structure Series, ten posts that might well change the way you perceive writing a novel completely. It sure did for me.
The Post #1 is here. At the bottom of each (past the comments) is a link to the next. Yes, they are long. You will need time to read them. Take it, if you never have.
I hope it proves as enlightening as it was to me.
*That, and Donald Maass' The Breakout Novelist. Don't make me choose.