Well, IDK if this counts, but if I can't sleep or whatever, and I just want to dream something, I shut my eyes and think of a story in my head. If I let that story in my head play out for long enough, I'll fall asleep and continue the story as a dream.
I do the same thing when I wake up right before something good happens in a dream. I'll try to remember what was just going on, shut my eyes again and play it out in my head, and sometimes I'll fall back asleep with the continuation of the same dream.
I usually don't have to do this though; I'm a dreamer already.