Have you taken care of your computer? Don't just log off, switch off. Turn printers, scanners, photocopiers, toilet lights, air-conditiones, etc. off when you leave.
Do you come to work in a tank top and change to your arctic gear once inside? Adjust the thermostat. If it's centrally controlled, ask for the temperature setting to be changed. Chances are your frozen coworkers are thinking the same thing.
Think before you print. Do you really need that email printed? Can you print it on recycled paper? Switch to FSC paper or recycled paper and save some carbon-absorbing trees.
Ask your boss to institute an energy policy. Set up saving targets. Ask for incentives! If you are the boss, set your staff some new performance targets!
Work in the comfort or your own home. Working from home one day out the week will save HUNDREDS of kilos of CO2 per year.