They were always a lot easier to see framed so you could trick your focus by looking at reflections in the glass. Ideal conditions: printed at poster size, at a shopping mall kiosk, near a trashcan with an ashtray.
I have ~-1.75 in one eye and -3.5 in the other and it took me a long time to eventually be able to resolve the hidden images. They usually aren't all that satisfying really.