For some reason, I can make my imagination way more vivid through a very unintuitive means.
Instead of focusing on imagining the colors and the other sensory perceptions of what I'm imagining (touch, smell, etc.), what I try to do is "visualize invisible objects".
So, objects that have a shape, but don't have any color, so you can't see them. And yet the objects still exist, and, paradoxically, make me perceive sensations (including visual too, somehow).
This works really well. Using my imagination in this way feels very fluent, I can imagine very intricate scenarios without needing to rewind them, or play them out in slow motion, and still get what feels like a very rich amount of detail.