Caulk everywhere the brick or stone meets the walls and ceilings. Cover the opening with tight-fitting glass doors. If you have an attic fan that you don't use, seal the opening with a temporary or permanent cover. A lot of heat can escape through the openings cut for recessed lights. Newer models can be covered with insulation.
Older models require a makeshift cover-like a bucket turned upside down in the attic.
Windows and Doors. Install weather-stripping on any that do not fit tightly. Insulate and weather-strip any entrances from your home into the attic. With a little detective work, you may find a lot of other leaks. Feel for air coming in through cracks and around windows and doors.