Close your windows properly. This includes ensuring that if you have them, storm windows are installed and closed in place. It should be latched to Windows. Open them during the day when the outside temperature is higher than the indoor temperature.
Keep the windows airtight. To seal them better, you might want to buy removable window caulk or plastic. Stuff a towel or jacket, at least, before any visible leaks.
Use cheap, transparent daylight shower curtains over the windows. This will keep out the cold air, and the sun’s warmth will heat up your house without cold air coming in. Clear plastic sheets can also cover the windows.
Put curtains in place. Heavy curtains will block heavy air drafts. Open them when the sun shines and cover them when they don’t.
Seal the doors. Check the door frame and under the door as well. You might want to buy a stripping weather or a sweeping door. Again, at least, make a dodger draft or stuff a towel at the door’s bottom.
Let your house hit as much sun as you can. Check for obstructions (e.g. plants, sheds) that could prevent the rays of the sun from reaching your home. Remove items on the sunny side of your house leaning against walls. (Ideally, put them back for extra insulation at night).
Open any rooms that are unused. The closed door makes the space into another wall between you and the outside frigid. It also prevents air from circulating as much, eliminating heat loss. Home improvement stores are selling magnetic register covers in unused rooms to “turn off” forced air furnace registers. That way, only the registers in the rooms you use will pump out heat when the heater clicks on. This makes use of the heater more effective.
Check for open adjustment of all heat registers, particularly where plumbing pipes can freeze. Unblock the returns of cold air in heated rooms (they can be blocked by furniture or rug) so that heat can circulate effectively.