I have tried an experiment the last week. When working on my house, I turned off the heater, because I had the doors open, and the unit kept going on. When I finished the job later in the day, I forgot to turn it back on. Later in the week, I noticed that it had been off, but no one complained (fairly reasonable weather though).
Last night the temperatures dropped to near freezing, but I left the heater off. The home's internal temperature went down to 68F from 72F (comfortable, but not warm) Everyone is fine. I think that this is due to the fact that I have added more attic insulation as well as finding ways to prevent the warm air from escaping the house.
I have posts on the blog like this one: Insulating the space above an attached garage and others that may give you some ideas.
Have you had success with insulation? It certainly will reduce my gas bill.