In elementary-school science class, we learned that warm air rises. As a homeowner or renter, keep that tidbit of knowledge in your pocket and make sure that same warm air doesn't escape from your home - right out of your chimney.
The U.S. Department of Energy reports that while you're getting cozy in front of a fire, your fireplace is allowing as much as 24,000 cubic feet of warm air from inside your home to escape outside. At the same time, cold air from the outside floods in.
A few simple tricks will help you avoid this. First, do you actually use your fireplace? If you answer "never," seal the chimney flue for the best stopper against heat loss.
If you do use your fireplace, be sure to keep the damper closed whenever you don't have a fire going — an open damper is like an open window. And check the damper seal to ensure it's tight.