Because the novel, Fangtooth that I'm working on has quite a few subplots, I'm having to spend a while plotting it out. Some of the subplots didn't appear until I was already writing it, so to stop me losing track of them, I'm having to work out the scenes first. Some people write novels off the cuff without plotting; others plot. My first novel, Evilution was written off the cuff, which is like flying by the seat of your pants because you don't know what to expect next. Now, I prefer to plot a story. Fangtooth started out with a basic plot, but now as it's becoming more diverse, I'm having to spend more time plotting the various threads. For me, plotting a story consists of a brainstorming session, imaginging a lot of 'what ifs', and then seeing which ones work best or seem more appealing. Eventually, an uncanny sort of rhythm seems to take over and things fall into place.
On another point, it's always a good idea to regularly save your work. As well as saving to a seperate disk, I have found it useful and reassuring to save my work to a hotmail account (made all the easier now they offer 250mb of space.) Now there's no worry about losing everything if the worst case scenario happened.