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This includes making sure storm windows are installed and closed in place if you have them. Windows should be latched. Open them during the day if the temperature outside is higher than the temperature indoors. Keep your windows air-tight. You may want to purchase removable window caulk or plastic to better seal them. At a minimum, stuff a towel or shirt in front of any noticeable leaks. This will keep the cold air out, and the warmth from the sun will heat your house without cold air coming in. You could also cover your windows with clear plastic sheets. A set of heavy curtains can block heavy drafts of air. Open them when the sun is shining and close them when it's not. Check around the door frame and also under the door. You may want to buy weather stripping or a door sweep. Again, at minimum, make a draft dodger or stuff a towel at the bottom of the door. Check for obstructions (e.g. plants, sheds) that might keep the sun's rays from reaching your house. Remove items leaning against walls on the sunny side of your house. (Ideally, put them back again at night for additional insulation). The closed door makes that room another barrier between you and the frigid outdoors. It also stops air from circulating as much, which reduces heat loss.  Home improvement stores sell magnetic register covers to "shut off" forced air furnace registers in unused rooms. That way when the heater does click on, only the registers in the rooms you use will pump out heat. This makes for more efficient use of the heater. Check that all heat registers are adjusted open, especially where plumbing pipes might freeze. Unblock cold air returns in heated rooms (they may be blocked with furniture or rug) so heat can circulate efficiently. Rugs and carpets help prevent heat loss through the floor. They are generally warmer to the touch than wood or stone, and so offer a warmer surface to walk on. A lot of heat escapes through the attic, as warm air rises and cold air sinks. Make sure that your attic has enough insulation. Cooking can help you to keep warm as an activity, through the oven's warmth and by eating something nice and warm after.  Bake cookies or a pie. Your oven will help to dry the air and heat the kitchen. The kitchen will be warm while you are cooking, and then you can have a great home cooked meal too! After, leave the oven on and open the oven door to let some heat in the house. Only leave the oven on for ten to twenty minutes, so you don't waste energy. Limit cooking that gives off steam, as this will increase the humidity in the air and make your house damp. Lowering humidity in the wintertime helps you to feel warmer. Water vapor (humidity) has a greater heat capacity (ability to absorb heat) than dry air. As a result, humid air feels colder in winter than does dry air and it takes more heat to make damp air feel comfortable. A candle/candles can produce a lot of heat, just be mindful of where they are placed and do not leave them unattended. A trip to most any grocery store or discount store can provide you with a number of candles cheap! Use a candle heater. It doesn't create as much heat as a fireplace or real heater, but will create warmth very cheaply. The average incandescent light bulb releases up to 95% of its energy as heat rather than light, making it an extremely efficient heat source. Compact Fluorescent and LED lights are not helpful in warming your room, so save them for warmer days and use the money you saved to pay the heat bill.

Summary:
Close all of your windows properly. Use cheap, transparent shower curtains over the windows that receive sunlight. Put up curtains. Seal your doors. Let as much sun hit your house as possible. Close off any unused rooms. Put down a rug or carpet. Add insulation in the attic and the crawl space. Cook. Light a candle. Turn on some lights.