You'll know the void when you encounter it. It is basically a loss of dream environment: it could be dark (mostly mine are dark), it could be white/light. It seems to be a sort of "holding place" where your dreaming mind is resting a bit from the work of dreaming before continuing. The most important thing is to realize that landing in the void does *not* mean that the dream is over. If you keep thinking about dream plans and maintain some movement (dancing, walking, flying), and think about where you'd like to be next, very often (most often with practice) your LD will continue and regain visuals in a few moments.

I agree on the earlier comments that movement helps, and keeping goals in mind is key. My best and longest LDs have arisen when I had clear, multiple dream goals and I kept them in mind in the dream moving from goal to goal. But I'd avoid jumping into movement without first having the first goal firmly in mind or you can risk losing lucidity. I think an initial moment scanning your surroundings, letting things stabilize, and affirming your lucidity before moving (too far or too fast at least) is very helpful.