Check windows and doors for drafts with a ribbon or feather. If it flutters, make or buy a draft dodger, or seal leaks with caulk and weatherstripping.
Click here for more information about the “draft dodger” and how to make one. You can achieve a similar affect by folding an old blanket to the appropriate size and placing it on the floor in front of the drafty door.
Daily action from http://www.lcwr.org/lcwrsocialjustice/eoclentcalendar2010.pdf