Turn off your computer at night or when you leave the office for extended periods.
A power strip that powers your computer, printer, router, and other equipment will allow you to turn off everything with the flick of one switch. Be careful, though, not to overload your powerstrip.
Remember to unplug battery chargers and power adapters when they finish charging or are not in use.
Turn off monitors when they're not in use. A CRT monitor (the old, bulky type of monitor) uses approximately 80% of the energy in a desktop system, and an LCD monitor (flat panel) uses about half that of a CRT. Even if you don’t plan to turn off the computer, consider turning off the monitor for a little extra savings.
Screen savers do not save power; they were developed as a way to avoid screen burn-in, and can actually cause a computer to use more power. Instead of using a screen saver, consider using sleep/standby settings. If security is an issue, OS X and Windows have settings that allow you to require a password on wake. On a Mac you can find this setting in System Preferences-->Security, and on a PC it's in Control Panel/Power Options Properties/Advanced.