How can I improve my ability to manipulate mental images?

When I try to visualize a scene in my mind I find that edges of the visualization fade away until I only have a tiny image in the center of my visual field or lose the visualization entirely.

Here are some things I have noticed:

• My ability to visualize seems to go through a cycle in which I first visualize a scene, then I lose pieces of it until it fades entirely, then a little while later I manage to reconstruct the scene.
• My ability to visualize seems better when I am near sleep though still lucid.
• Rather than fading evenly towards the center I find that I lose entire chunks of peripheral vision at a time.