The US Department of Energy estimates that drafts cause 5-30% of heating costs to be wasted. You would not keep your door open all winter, yet the Earthworks group estimates that the leaks in the average American home, if added together, would amount to a 9 Foot hole in the wall.
Finding and sealing leaks is a very easy way to save money. To find leaks, simply light an incense stick and slowly go over baseboards, windows, and doors on a breezy day, and watch for the smoke to move. Putting caulk on these spots is a simple fix to the problem. If your windows are old, consider replacing them with newer, more energy-efficient models. The Federal Government offers tax credits for projects such as these, see the EnergyStar Website for more info. If replacement is not in the cards for this year, consider purchasing window shrink-wrap. It may not look appealing, but your wallet will thank you.