According to Energy Star here are some of the top improvements that homeowners should typically make after an audit of their home.
Air Sealing and adding insulation- Seal up small holes in your home, especially attics and basements.
Heating and Cooling- Systems more than 10 years old should be replaced with a newer Energy Star Rated System.
Ducts- Seal and Clean your Duct Work.
Lighting and Appliances-Replacing older bulbs and electronics with newer, more efficient models can save you hundreds and reduce greenhouse gas emissions