I used sticky back pads on our 2.5 x 2.5ft mirror. It fell during the night, a week later, but luckily it didn't break.
I took all the old sticky pads off, replaced them with fresh pads, then applied no nails to mirror as well. The idea being, is the pads will hold the mirror to the walls, while the no nails sets.
Three years on, and the mirror is still on the wall.
I'm dreading taking it off, though.