We installed a programmable thermostat years ago and saw our gas bill drop a third. One of the things I noticed was that when the thermostat was not programmed to run, the temperature would fall fairly quickly in the house. I did a quick walk through each of the rooms and I could feel a draft in almost every one. The next fall I set out on a mission to close all those cracks. I spent one weekend caulking windows and installing weather stripping on the doors. That next winter our heating bill was half the previous year, it was a milder winter and gas wasn't quite as much, but looking at the actual gas usage we used 30% less natural gas. The house doesn't cool down nearly as fast now, and the $20 I spent in materials has already been saved at least 20 times. If you have some extra time this weekend, think about air sealing your house. Here is
a guide on air sealing to get you headed in the right direction.