That always happens to me, too. But what you can do is learn to anticipate it. Next time you become lucid, relax, and become aware of how you're feeling in the LD. When you feel everything going black, keep reminding yourself that you're dreaming constantly and be patient until things stabilize. You can also try visualizing an image until it shows up.

I would also try remembering what position your sleeping body is in. In my last LD I was afraid that I would wake up in the same position that I was currently dreaming myself in (sitting up), but when I woke up I realized that that wouldn't have happened. Just try to keep the feel of the dream in your mind and relax.

Piece of advice, though, don't open your eyes.