One of the quickest, best ways to save money is to weather-strip, caulk, and seal all of the seams and cracks in your home. Reducing air leaks can save you 10% of more on energy costs.
Robin, that's the quickest way to get sick. You DO need fresh air to breathe and to kill the mostly anaerobic bacteria.
The trick is to control the drafts, not stop them. Provide a controllable air inlet near the furnace. Otherwise the furnace will suck cold air in through door and window cracks or sometimes even the walls. Ideally, an air intake at a low spot should provide enough air so that any window drafts are to the outside. If a candle flame near a window or door frame points INTO the house, then the furnace burns paid for, heated air and runs inefficient.