Sure, things like turning off the lights when you leave a room seem completely obvious, but those are precisely the things we often overlook the most. What other things in your office draw power, even when they're not in use? Something as simple as turning off the copy machine at night can make a big difference. If it only needs to be used eight hours a day, why spend money to keep it powered up for the other sixteen?
Many people don't realize that most electronic devices draw power even when they're not being used. Even if your computer goes into "sleep" mode after it hasn't been used for a while, for example, it is still drawing power. Shutting it completely down at night can save you a huge amount of energy. Whether it's on or not, televisions and other devices that work by remote control are also using electricity in order to stay in a "standby" mode. Solve this problem by unplugging them when not in use.
Finally, electronics chargers also use electricity, even when nothing is being charged. When you take your cell phone, digital camera, or pda off of the charger, make the extra effort to unplug the charger, too. It's estimated that 40% of the power needed for chargers is actually used when they're not even charging anything. Consider plugging your chargers into a surge protector so you can simply flip the "off" switch once your electronic goodies are all charged up.
It doesn't seem like such a small thing could make such a difference. But if you could reduce your energy bill by 30%, that has the potential to add up to a lot of savings. Not only that, but it's the responsible thing to do.