[header] How to make a fireplace more energy efficient [title] Install a top-sealing chimney damper. [step] Top-sealing dampers replace the fireplace throat damper and are installed at the top of the chimney. The top-sealing damper has a seal that acts like a storm door, keeping the expensive conditioned air inside the house and the outside air-outside. 
This principle works year-round, whether you're heating or cooling your house. This product can be purchased online and is easily installed by either a homeowner or a handyman.