Turn off your computer monitor, printers when you go to lunch and when you go home.
Turn off the lights in your office if you're the last one to leave.
Making a drink? Only boil the amount of water you will actually need or better still why don’t you upgrade to THE ECO KETTLE with its patented “Dual Chamber” design, the ECO Kettle is different. The internal reservoir holds 1.5 litres of water ready for use, while the measuring button allows any quantity to be released into a separate chamber for boiling. Simply fill up, then decide exactly how much water to boil from one cup to full capacity at the touch of a button.
Now recommended by the Energy Saving Trust
Lower the room temperature by adjusting heating rather than opening windows
Use the stairs instead of the lift as much as possible.
Generally keep your eyes peeled for lights left on when they don't need to be, electrical equipment on standby etc
Apple and Dell have a useful energy usage guide, these should give you an indication of the power consumption of their products. Unfortunately these comparison charts are only available in US dollars at the moment!