Mirrors reflect light, so the more mirrors you have in a room, the more light will bounce from wall to wall! Hang mirrors opposite windows or glass doors to maximize the natural light in your room.

Angie's How To Increase Your Home's Natural Light
If your living room receives cave-like natural light, you're going to be flicking those light switches a lot. When you turn on the lights, you use electricity – and that costs money! Save a little cash and reduce your energy usage by increasing your home's natural light. Plus, you'll get a mood boost from all of that vitamin D!
3 pros!
Tip: Buy furniture with glass panels or mirrored accents.
Light colors reflect sunlight better than dark ones, so a wall painted in cool tones (like gray, cream or eggshell blue) will boost the natural light in the room.
Unless you have some kind of translucent plastic theme going on, your furniture is blocking natural light! Move furniture at least a few feet away from any windows or glass doors so you can let the light shine in.
It's important to have drapes and curtains, but they don't need to be made from heavy fabrics like velvet or brocade (which will make the room feel darker and smaller). Try linen or cotton, instead!
If you have a solid, exterior door in your dark room, replace it with a door that has a window. If you're worried about burglary, you can find glass doors at your local hardware store that have high-security glass.
Some shades, like Roman shades, block sunlight even when they're open! Unless you're trying to keep glare off of a TV, this isn't a great choice for darker rooms. Instead, opt for blinds that let you better control the amount of light that comes in.
This is a pricey option up-front, but because a skylight can help naturally heat a room, it could save you boatloads on your electricity bill over time! If you're ready to take the plunge and add a skylight to a room, hire a contractor for the job – don't try this one yourself!