Check The Insulation in your Attic.
If you have an unfinished attic, pop your head up there and take a look around. You should see insulation up there between the beams, and there should be at least six inches of it everywhere (more if you live in the northern part of the United States)
If there is inadequate insulation up there--or the insulation you have appears to be damaged--install new insulation. Also, depending on where you live some states offer financial incentives, up to a 75% refund for instance, to encourage homeowners to better insulate their homes.