Once the vibrations set in, imagine yourself rocking back and forth while staying totally relaxed and you will eventually find your visualization translated into the sensation of motion. Increase the rocking until you turn completely over.

Recently, I've found that I can turn over continuously until I'm spinning like a top and then just treat it like LaBerge's "spinning to prolong lucid dreaming" technique. I spin while repeating to myself "I'm dreaming." and soon find myself in a lucid dream.