"Pepper's Ghost effect" was named after a man named John Pepper back in the late 1800's, who devised trickery on stage using mirrors and reflections to give ghostly appearances during stage productions. Disney used the same idea when the Haunted Mansion was created. In the ballroom scene, people on the ride see reflections in glass that make the dancers look ghostlike as they turn about the dance floor. Disney also used a material called scrim (which can be painted) with special lighting effects in its Carousel of Progress ride and the Country Bear Jamboree Show, so that objects and subjects could appear and disappear magically when front and back lights were switched on and off.
Edge lighting - a bit rough, but the technique seems usable for signs?
Videos
Miniature printables
Wallpapers
Other Printies
Blogs
Groups
Supplies for sale
Hobby Lobby - buy a spray can of "Looking Glass" by Krylon (#9033). You spray several very thin coats on plain glass to give you a reflective 90% mirror affect.