No matter how many space heaters you have or how hot your radiator is running, if your home isn’t properly insulated you are in for a long winter. The first place you should star with are your doors and windows.
Weatherstripping Windows and Doors [Lowes]
Weatherstripping Your Home [Department of Energy]
Installing Door Weatherstripping (video) [Expert Village]
Weatherstripping a Window [Ace Hardware]