To increase the amount of natural light projected into a room, simply place a mirror in front of or next to a window; no matter how shady the sky is outside, the mirror will still make use of the available daylight. The larger the mirror, the brighter and wider the perceived space will be.
Remember, however, that it is not only natural light that can be manipulated in this way; mirrors are also great for emphasising the output of other light sources. If you need to furnish a rather dark space, placing a mirror next to a table lamp can make all the difference.
Be careful, however, not to place the mirrors directly in front of the lamps, especially in the presence of spotlights, as the reflection could be annoying: prefer side positions, well spaced or at least creating indirect reflections.