If I'm using solely my imagination... My sexuality tends to be very primal, so I tend to have primal fantasies when I masturbate. Earthy scents, like the smell of an old coniferous forest will sometimes prime me. Or a mountain lake. A lot of my fantasies involve control and establishing dominance purely with physical presence (my primal side again). When I take hot showers, using the shower head, unf! There's a lot that I've found to get me going, things I do during, but I'm still trying to feel out the boundaries of how explicit is okay on this forum before I delve further.