Just add blankets to the bed and sleep in heavier pajamas. Most heat is lost through doors and windows. Teach family members not to leave doors open unnecessarily long. Add weatherstrips around doors, lock windows for tighter seals and inspect for caulking needs. Heavy gauge plastic can serve as a temporary storm window during the winter months.
Drapes can provide additional insulation.
They don't have to be heavy. Keep drapes closed for insulation on the north side of the house where sun will not be coming in during the day. Let the sun shine in on the south side during the day, then draw the curtains at night. Poor ventilation will increase the chances for mold and mildew. Close doors to those rooms instead.