My first experience was by smoking. It was in a dark room (but during the daytime) with some calming music, sitting upright on a couch, and after about 10 minutes of meditation.

After smoking, the floor seemed to change form, it still looked like carpet, but it started to move with waves and ripples like it was a liquid. When I looked up from the floor I noticed across the room the music from my computer speakers was causing the air to distort, almost like I could see the sound waves. Then it got really interesting, I tilted my head back to look up at my ceiling — but it was completely gone. I could see right through the ceiling to the sky above. Using After Effects immediately afterwards I was able to recreate almost exactly how the sky looked. Here's the video: http://www.youtube.com/watch?v=Vjjw825IWf8