Before we can get into all the great tools, articles, and nitty-gritty prewriting stuff, it always helps to start with something a little more general. Like this article at the KU Writing Center about prewriting and the most common forms it comes in.
For NaNoWriMo, there are a lot of people that skip prewriting except in its most basic forms (like brainstorming). But everyone should do at least a little. And for people who find themselves getting stuck in the middle of a scene, an outline can be incredibly helpful. I find that I often don’t want to write in a completely linear fashion and an outline makes that something that I can do without losing track of my story.