I used to be better at it, back when I was doing a lot worse than I am now. Back when I took shelter in my head, I could lie there for hours, thinking, 'til my limbs went cold.
Another thing that gets in the way is the direction I am facing while I think. I'll do my best to describe this...
If I happen to change position while I'm visualizing, the image(s) remain in the same orientation. For example, If I were to be looking straight ahead, then crooked my head to the side, the image from before would appear sideways in my mind; it did not change its position. Just like when you look at a picture, computer screen, etc., if you turn sideways, that image will now appear sideways. You are the one who has moved, not the object.
It's like my mental images are physically removed from me...