A fun visualisation trick
You can use memories of specific actions to help control your mental images with much less effort than simply willing them to do what you want. For example:
If you recall the feeling of dragging the saturation slider in an photo editing program to the left, you find that whatever you're picturing magically becomes greyscale. Or, if you see a black-and-white photo and want to imagine it in colour, close your eyes and imagine dragging the saturation slider to the right.
I found it entertaining (and surprisingly easy) to mentally rearrange (or add and remove) the walls and furniture in a room, or changing a person's appearance (hairstyle, hair colour, etc.), by pretending I'm playing The Sims.
I also used to use the memory of adjusting a microscope focus knob on a blurry picture until it's in focus as a way of getting sharper mental images.
Another version of this trick is prompting yourself with a broad genre of imagery, like telling yourself you're watching a movie trailer. I find this less effective than more specific forms of intention but it's good for, say, reducing intrusive images.
This is inspired by lucid dreaming techniques that use 'mental schemas' to control dreams. The same principle applies to waking mental imagery. I imagine you could use this trick to train yourself to get very good at specific transformations, like using a video game character creator to get good at visualising people in different ways.
