Mirrors increase and reflect light sources. Use mirrors liberally in rooms that lack natural light.
Try using a freestanding mirror at the end of a hall. It brightens the whole hall, reducing that feeling of walking into a hole
Layering mirrors offers a unique tabletop vignette. Start with a large mirror hung on the wall. Lean a smaller mirror on your table top against the wall mirror. Finish by layering even smaller mirrors in front.
Place lamps on tabletops in front of mirrors. The light if reflected, offering a warm glow.