It helps me immensely to have a plan, an outline, or just a rough sketch for my story before I actually sit down to write it. You're not allowed to write any prose until the first of November, but you can certainly write outlines, timelines, histories, or character sketches. If you know you want a scene where the protagonist unexpectedly bumps into the antagonist during a spring downpour, then scribble a note down to that effect. If you know they'll be discussing a certain topic, mention it in your note. But don't write any of their dialogue, nor any setting descriptions that you'll be tempted to use in the actual novel.
In the few years I've taken on the NaNoWriMo challenge, I've discovered that somewhere in Week Two or Three I invariably end up at a 'well, now what?!' moment. I've used Week One's head of steam, I've stretched into characters who were less three-dimensional and given them some motivations and background, and I can't think of anything else to actually write about that will be useful in the story. In NaNos past I've been plagued with the problem of being able to write something, but not what I need to write to advance the plot and get to the end. It's because I don't know where the end actually is. With a plan or outline in my hand, I have a road map to the various points of interest in the story. It's like a built-in tour guide, defining the important stops along the way. And even if all you do is scrawl down a few index cards' worth of scene notes, you'll have something to help get you back into the swing of things when the plot winds down. To continue the example I mentioned above, you might grab a 3x5 card and write: "Sally and John meet by accident in the parking lot of the mall in a spring downpour, argue about last summer's disastrous company picnic". Then file it away or tuck it in your writing notebook (don't lose it!) and take it out when you run out of story. It might just save your butt.