Use compact fluorescent or LED bulbs. Replace three frequently used light bulbs with compact fluorescent bulbs and save 300 lbs. of carbon dioxide and US$60 per year. A standard compact fluorescent bulb will save around one third of a tonne of greenhouse gas, along with the cost of six or more incandescent globes.
Remember that CFL bulbs do contain small amounts of toxic mercury. Therefore, proper disposal (recycling) is necessary to prevent any additional landfill contamination.
You can also start looking into LED lightbulbs which have started to crop up recently -- they are even more efficient, longer lasting, and do not contain mercury!
Consider substituting as many energy efficient bulbs as you can, and give them as gifts to family and friends. Donate a set to a local charity to refit their office.