unrepentantAuthor , the possibilities are very much endless. It's hard to create a world like you want it designed, but you can give your mind general clues. Music also works; one example:
It's only the Fairy Tale-Lyrics - YouTube
See the lyrics, you don't need to listen to the song (lulz, horrible japanese-english)

The mention of "dark castle" and "dark side of moon" made me imagine moonlit night, in a, surprise, surprise, dark castle.
When it came to "dancing in the shadow", due to the variations in music, I suddenly found myself flying above it all.

And so forth, the possibilities for inspiration are endless; and which lyrics/types of music triggers what experiences is probably personal (often it's based more on associations and metaphors, as dreams usually are). Watching a movie before going to sleep often makes elements of that movie appear in your dreams, convoluted or no.

As for how to trigger it? It may vary from person to person. I discovered it all by myself a few years ago, long before I knew of the concept lucid dreaming at all. I did it after going to bed once, and that is the first requirement, you need to be somewhere you can relax, both your body and your mind. The trick is to get your body to "fall asleep" (almost or entirely), and let your mind only almost fall asleep (leaving it rather blank and open to interpretations. You've probably experienced that when you go to bed, you start getting really creative or find easy solutions to problems you've had before, right? This is because when you try to sleep, you automatically start to "blank out" your mind, mentally relax through not really thinking about anything.

Trying to replicate this blanking out, but going a step further, is the first step. I'll describe how I felt the first time I did it: I started by, of course, relaxing, blanking my mind; I tried to imagine that I "shut off my brain" (by imagining some sort of block being placed in my neck, on whatever connections the brain has to the rest of my body. I closed my eyes during this, and the harder I tried to shut off my brain, the more I started noticing that my eyes started moving by themselves, back and forth. Don't resist this, try and let your eye muscles relax, continue trying to relax; also try to imagine your brain and head as a "muscle" that can be relaxed, to let everything happen on its own. Try to remove your awareness (but not so much that you risk falling a sleep; you must have at least miniscule awareness of being awake). If done "correctly", you may start seeing images appear before you. Try to relax still, and don't focus on what's happening; that ruins the process. Try and let your consciousness "float", let it be like a continuous stream that you're flowing with, not resisting in the slightest. Then before long you might find yourself dreaming. It always requires practice; at first you will have to concentrate too much on the various tasks to be able to relax enough, but get used to it, and it'll come more automatically.

I find it most effective right before sleep, or after having just awoke - in the first condition it is important that you slept well the night before, and in the last one it is important that you still have that feeling of being "half awake", also, if you wake up while in the middle of a dream, it's much easier.

Of course it's all subjective how you enter, or more specifically, how you feel while entering, but it might be of help ;)