Change out dim light bulbs for bright ones. Fluorescent ones in particular can multiply lumen output without increasing your energy usage.
Paint the walls of the room white or another light color. Black paint absorbs all colors of light, and other paints -- green paint for example -- reflects only green light. However, white reflects all colors of light. The lighter the paint, the more light it is reflecting, amplifying light output in a room.
Hang a large mirror so that it will amplify natural light coming from window or skylights, or artificial light coming from a lamp or other light fixture. As an added bonus, mirrors open up rooms to make them look bigger.
Change lampshades and window dressings to let more light through. Lampshades are designed differently to allow different amounts and qualities of light into a room. While you may like the mood of a thicker or frosted lampshade, a thin or more transparent one will promote any lightbulb's light output. Heavy curtains will curtail sunlight, so switch to translucent muslin curtains that will afford privacy and allow light in.