I think what Alex says is true for me too - when I stop relating what I am seeing the "window" fogs over. That's when I get the swirling greys and fancy whirls, mostly on the back of my eye lids.
I play the "where shall I go this time" game, if you will. I go with whatever scene first comes to mind, and not decide that I don't want that one. It came for a reason, so go with it. Then I just start imagining things there, or action happening. Go with whatever comes to mind - if you speak it out loud you really don't have time to decide in your head that nahhh... I don't think that's an important image. 
The more I let the scene play out, the more that happens. After I get going there is little repetition. I am to the point where I am starting to see reoccuring patterns - where my mind/thoughts tend to wander to. And then there are those sudden flashes of a single image that just pop in by themselves.
Now to put them all together and figure out what they mean!
*A* fellow traveler
I'm getting closer!
