Open blinds and curtains during the day and close them in the evening.
Don’t put furniture in front of heat sources (baseboard heaters, radiators, etc.).
If you have forced-air heating, keep air intakes and registers clean: the system will work better.
When you use your fireplace, as soon as the fire has died down, shut the doors and damper (to close the chimney flue) to keep warm air in and cold air out.
Clean ventilation system filters regularly and replace them as specified by the manufacturer. You’ll optimize the air flow.
Make sure that the hatch to the roof space is airtight and well insulated. If you can get into your roof space, caulk any gaps around pipe penetration points, light fixtures and other such openings.