Replace old incandescent light bulbs with energy saving bulbs where you can in your house and start saving energy now. Did you know that compact fluorescent light bulbs (CFLs) use around 20% of the power of incandescent bulbs to produce the same amount of light, and light emitting diodes (LEDs) use even less? Modern energy efficient bulbs are designed to fit into existing light sockets and can produce just as warm light as incandescent bulbs. Whilst they may cost a little more than incandescent bulbs to buy, they save a lot of energy and last much longer before they need replacing (years or even decades!).
Also, don't forget to turn the lights off when you leave the room!