1. Closing doors and windows and keeping them air tight will lock in any heat. Use a towel or similar to block any noticeable leaks. Weather stripping could also keep the cold out. Heavy curtains can block drafts, but open them when the sun is shining to let heat in.
2. Closing off any unused rooms reduces air circulation and therefore reduces heat loss, and putting down a rug helps to prevent heat loss through the floor.
3. Cooking a meal in the oven or on the stove can help the chef keep warm, dry out the air and heat the kitchen. After cooking, open the door to let some heat into the house. Limit cooking that gives off steam. Water vapour absorbs more heat than dry air, so humid air feels colder in winter than dry air.