Print out old family photos. If they aren't already black and white or sepia toned, change the color of the photos before printing. No inkjet - use a laser printer or print at a store like Kinkos.
Trim photos to fit the mirror border and attach with adhesive. Add old buttons, watch parts and scraps of leather with E6000 . . .
Cover the mirror and apply matte decoupage over the bordered photos and you're done! . . .