Seal fireplaces that are not used (a makeshift foam plug can be inserted and then removed if you decide to use it again). Hanging a flag from the plug will serve as a safety reminder. Remember even a closed damper leaks a large amount of heated air to the outdoors.
Avoid having your furnace thermostat in a room with a fireplace.
Consider installing tempered glass fireplace doors and a heat-air exchange system so warmed air could be blown back into the room.