If you never use your fireplace, plug and seal the chimney flue.
Keep your fireplace damper closed unless a fire is going. Keeping the damper open is like keeping a 48-inch window wide open during the winter; it allows warm air to go right up the chimney.
When you use the fireplace, reduce heat loss by opening dampers in the bottom of the firebox (if provided) or open the nearest window slightly-approximately 1 inch-and close doors leading into the room. Lower the thermostat setting to between 50 and 55 degrees F.
Install tempered glass doors and a heat-air exchange system that blows warmed air back into the room.
Check the seal on the flue damper and make it as snug as possible.
Add caulking around the fireplace hearth.
Use grates made of C-shaped metal tubes to draw cool room air into the fireplace and circulate warm air back into the house.