The mirror in the most recent revival of Three Tall Women deserves a mention, a fantastic way to denote what’s happening in the second half as well as being just plain surreal. For those who missed it, the first act (with the three “real” women) took place in a lavish bedroom that occupied the front half of the stage. For the second act (where the women become the old woman at different stages of life), the wall at the back of the room flew up, revealing an exact mirror copy of the set facing upstage, and behind that was an actual huge mirror, reflecting both the mirrored set and the downstage set, which is where the action continues to take place. On top of that, in the bed in the mirrored set was an actress playing the (dying, I think) old lady, who mostly laid comatose but would move now and then.
It was so simple but so perfect.