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Watch the center black dot without moving your eyes at all. After a while the surrounding colors of fluff vanish. That's about the level of stillness I've found is needed in order to develop WILD imagery and keep it.
Try practicing to make the imagery to disappear, then start counting to see how long you can keep the imagery gone. When you are going to sleep or are visualizing, try to keep your eyes that still for as long as you can.