Caulking around windows and
doors is a low-cost solution.

4 More Ways to Keep Warm Air Where It Belongs

7. Stop air movement. Close the fireplace damper when it's not being used. Check your dryer vent cap to make sure it closes tightly when it's not being used. Use ventilating fans sparingly.

8. Plug up the big gaps. Seal cracks, especially around windows and doors, with caulk and weather stripping. Install storm windows.

9. Plug up the small gaps, too. Put foam insulators behind the faceplates for light switches and electrical outlets.

10. Add a barrier. Install or increase the amount of insulation, paying special attention to the attic and basement where the biggest gaps and cracks are lurking.