I like to come up with stories like movies in my head. I'm out within 5mins unless I come up with something I particularly like, then I'm trying to keep myself up so I can keep watching.
I sometimes let my mind go on a fantasia-esque dream journey. I always start with the same imagery, and then try to coax my subconscious into taking the reins. It's like tricking your mind into the REM state.