Heat can escape from your house in many unexpected places, through electrical outlets, attic entrances and furnace flues. It is more likely, however, to sneak out of the likely places like the doors and the windows. You could replace your door with an extremely expensive insulated door or have state-of-the-art windows installed, but that would cost you tons of money. An easier way to reduce heat loss is with weatherstripping.
Weatherstripping is an inexpensive alternative to out-and-out replacement or renovation. The materials can cost as low as twenty dollars. Of course, there is high-end weatherstripping, but you should weigh the cost of the weatherstripping vs. the amount of money and energy you will save. According to a Cornell webpage, weatherstripping and caulking can save you up to 20% on your annual bills. That's a decent savings, but by choosing a more affordable weatherstripping instead of a top-tier pick, you're probably not going to affect that number much.
It only takes an hour or so to install weatherstripping . You can buy it in an easy-to-use kit or you can buy the basic materials and put the weatherstripping up yourself. Check out How Stuff Works for information on how to install weatherstripping.