Relaxation exercises really help. You could try lying on your back until you feel like you're "stuck" to your bed, or really heavy, then roll over onto your side. If you sleep on your stomach, you can then roll over onto it once you're even more relaxed from being on your side.

Don't eat foods with a lot of caffeine or carbs before bed, like bread, chocolate, pastries, and DON'T drink energy drinks, obviously. On a more serious note, imagining a scenario in your head (also known as daydreaming) for your next lucid dream works extremely well for some people, including me. Imagine what you'll see and do in your next lucid; watch the scenario like a movie and not from a first-person angle.

Sometimes I end up imagining the same scenario over and over again, which helps me fall asleep even faster! It doesn't get very boring either, because I'm always making minute changes to it inside my head.