Well it is easier the closer you are to sleep, so practicing during a wbtb could show some big improvement that might let you know what you need to get better. I don't really see visualization as something that comes quickly. I have been doing it since I was a kid, so I can improve pretty easily, but to get better than I have been before takes months of practice. In order to get back to where I used to be it takes like a week.

Do you alternate between things that are easy to visualize and things that are hard to visualize? That is a good exercise.