Pull the shades.
Yep, it’s that simple. Window coverings help hold heat in and cold out.
Close your fireplace damper and doors.
Only close up when you don’t have a fire lit, and it’s a good idea to let the fireplace cool completely before closing the damper.
Get your fireplace a pillow.
Yes, fireplace pillows or chimney balloons are a thing. They inflate in your chimney to block drafts (and noise, soot and debris).
Install weather stripping around doors.
Seal gaps and keep drafts at bay.
Add or repair insulation in roofs and walls.
Not an easy fix, but the benefits in energy savings could offset the work and expense.