Remove or cover window air conditioner units when it starts to get cold outside, then weatherstrip the window to seal any cracks. If using a quilted cover, be sure to replace any side seals where the AC fits in the jamb.
Cost: About $18 for a cover.
Savings: About $100 off your heating bill for three ACs.
Bonus: Weathertight rooms stay cleaner because fewer outdoor pollutants get inside.