Check Your Insulation
"This is one of the easiest ways to control your energy costs. Effective insulation slows the rate that heat flow out of the house in winter or into the house in the summer, so less energy is required to heat or cool the house. If your house has no wall insulation, and it has more-or-less continuous wall cavities (such as conventional wall studs), blown-in insulation can greatly improve your comfort and save enough energy to be very cost-effective," explains Justin Krzyston. Check out this article on choosing insulation.