It seems that I map out my entire story, get to the second chapter, then trash the map completely and write whatever comes to me. I still try to make the main events happen, so I guess it's more like guiding the story through these checkpoints, rather then me driving the story around under complete control.
As far as dreams go, I think they could be great for story ideas. In fact, I try to see if I can twist my dream around to see if I can make an original story out of it. Unfortunately, I have a very active imagination and it's hard to make a story that makes sense when your dream consists of driving a monster truck through your old high school's marching band camp, into the new band director's house, and then flying away on a dragon to a moon made of jello. XD