One important part of winterizing your home is to check for air leaks. Even a small amount will affect your home's heat retention. I remember when I started testing around the house for air leaks, I was amazed that even an electrical outlet gives out a lot of draft. We purchased the kit to put in it to reduce the draft. It lessened the draft considerably but I wasn't satisfied so what I did was to put on a strip of packing tape on it. Here are some places to check for drafts. Basically check all areas where there could be cracks or gaps.

Door and window frames
Mail shutes
Electrical and gas service entrances
Cable TV and phone lines
Outdoor water faucets
Where dryer vents pass through walls
Bricks, siding, stucco, and foundation
Air conditioners
Vents and fans.