I’d like to think that everyone is already making this a habit, but experience has taught me otherwise. You can save a lot of money by turning off your lights when you leave a room. Or the television, or stereo, or other electronic device. Or powering down your computer instead of leaving it on when you walk away for more than ten minutes. (Though if your computer takes 30 minutes to boot up, like mine used to, you might think twice about that.)
In addition to turning off the lights, consider replacing your light bulbs with more energy-efficient ones as well. I don’t recommend compact fluorescent bulbs because they are a health and environmental hazard (they have mercury inside, and it is the rare person who bothers to dispose of them properly; also, if a bulb breaks in your house, now you have fine mercury dust in your home). But the new LED light bulbs, though outrageously expensive, will last for thousands of hours, so it might be worth considering one of those to replace a light bulb in a fixture you use every day. Or start the transition by buying lower-wattage incandescents.
Another thing you can do is plug a lot of your appliances and electronic equipment into a power strip. Many electronic devices use electricity even when they are turned off. If you have them plugged into a power strip, when you aren’t using the device, turn the power strip off. That prevents the device from drawing residual power. That alone can save a lot of energy. I don’t recommend this if your device has settings that have to be redone every time your device is completely unplugged, which is what plugging it into a power strip, and then turning off the power strip, is essentially the equivalent of. But if you can stand your device flashing 12:00 at you all the time, then you can save a lot of money this way.
It may take some time to develop the habit of turning off the lights and other electronics when you leave a room. And it can cost a bit (and therefore take some time) to switch over to LEDs. Common sense, not to mention common courtesy, suggests that turning off the lights is the job of whomever is the last to leave a room.
You will be amazed at how much less your electric bill can be if you make these practices a habit.