Try visualizing a relaxing scenario. It should be relaxing, as an exciting or dangerous scenario would be counterproductive to sleep. The Secondary Visual Cortex is the section of our brain that functions during visualization and dreaming. This is not theory but a scientific fact. One of the first things our brains do upon entering deep sleep is begin non-REM dreaming, so by using visualization you are stimulating the same area of your brain as when you are dreaming and if you can do that you should be able to coax more lucid dreaming experiences.

Good luck!