Keep your thermostat at a reasonable temperature.

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Programmable Thermostats
Curb Your Temper(ature)
Every degree you lower the thermostat in winter and raise it in summer can reduce your heating and cooling bills by 1 percent or more. Install an automatic, programmable thermostat (they typically cost $40 to $100) that allows you to set specific temperatures for different times of day. For example, in winter you don't need it as warm when you're sleeping, and in summer you don't need to cool the house while you're at work. According to the U.S. Environmental Protection Agency (EPA), the device can save you about $150 a year.

While on the subject of temperature, make sure your water heater is set at the mid-range of 120 degrees; anything hotter and you're wasting energy. And if your water heater is at least three years old, wrap it in an insulation blanket (newer models are better insulated), which costs less than $10 at your local hardware store and will save enough energy to prevent the release of up to 1,000 pounds of carbon dioxide, as per