Correctly sealing a fireplace chimney depends on its inherent problems. For most masonry chimneys, the culprit is a drafty flue and/or inefficient damper. Even when in use, old chimneys tend to suck out as much hot air as they generate. Worse yet, when not in use, warmth generated by central heating is wasted if the chimney isn't properly sealed.
There are a number of ways to prevent house air from escaping out the fireplace chimney or to keep cold air from coming in. When not in use, filling the flue with an inflatable plug, other draft stopper or tightly-rolled piece of batt insulation works. You can also use weatherstripping or high temperature-resistant caulk to seal any gaps around the masonry fireplace doors.