Install more attic insulation. The recommended amount is 18 inches.
Check for gaps between the bottom of the door and the threshold. If there is one, install a vinyl or rubber door sweep.
"Anything that keeps those drafts, the cool air from coming in, but again, using the foam insulation down along on the sill of the door, around it, is a great way, again, to cut down on those drafts coming in," says Pete Seyfer of Alliant Energy's "PowerHouse" television program.
You can also install foam gaskets behind all outlet covers and switches.
And check for leaks in your heating ducts. Hot air can escape through the joints in the ducts. You can use duct tape to seal those ducts up.
Caulk around window casings and use weatherstripping around the jambs.
Make sure you cover the window with plastic window film, too.