Look For Natural Patterns
Everyone’s brain works differently, so there are no guarantees, but there is a horse grazing, with a foal running below in this image, taken in Titus Canyon of Death Valley. It isn’t subtle at all, yet I find that some people have trouble seeing it, which seems weird to me.
The actual subject is blue and white marble of a canyon wall.
I actively look for such images, waiting for them to appear out of patterns. I find that sometimes I’m first subconciously aware of an image being there, and it can take some time for it to emerge into my conscious mind. Shooting upside-down, as with a view camera ground glass can also help.