What is the best way to keep the cold outside and the heat inside?

You can buy cheap window insulation kits at Walmart, hang blankets on the walls, consider replacing your fireplace with a wood stove (you can pick up used ones and save money) or try a programmerable thermostat. If you are low income many states have home weatherization programs to help. Some replace roofs, add insulation and new windows at no cost to you.

Thermal drapes and/or cellular shades would hold more heat in, as would those plastic window film kits on each window.

Switching out ceiling fixtures for ceiling fans with the reversible switch allows you to blow the rising hot air down into the room.

Adding more insulation into the attic would improve heat retention. Easily done in a weekend.

Get insulation kits for your electrical outlets.

Caulk around all windows.

Call your power company for a free energy audit with a thermal camera they can pin point all the areas you're losing heat.

Add foam insulation sleeves to pipes and heat tape if needed. Wrap your water heater in a heat blanket and insulation the hot water line.

Add insulating weather strips around exterior doors, I prefer the metal with the rubber gasket myself. Make sure there is no draft under door also.

You can always use the old shrink wrap plastic for the windows. you can buy it at Home Depot or Walmart just about anywhere. you can use the self-adhesive foam around the doors, and use the foam inserts into your wall plugs. it's amazing how much air comes in through those wall sockets. Hope that helps

Line the walls with Polystyrene or foil backed - If cavity wall, insulation can be pumped in through tiny holes...Thick Curtains. Wooden shutters

Unbelievably, I had a friend in a house locally whose furnace return air went up to the ceiling, and was never connected, so vented into the attic and out. If that wasn't bad enough, eventually her supply ducts fell apart under the floor, and the house became impossible to heat. So, you might look over your ducts. Also, if your roof is melted over the house but not the eaves, there is heat leaking into the attic. Or, someone vented the bath fan into the attic (bad!).

My pleasure - Keep warm...