Some very easy ways to increase your fuel efficiency:

Clear the snow: lf you live in a cold environment, snow on top of your car is a familiar thing.  Be sure to clean as much of it off of your vehicle as possible before driving.  Snow adds both extra weight and more drag (air resistance while driving) to your vehicle.  Pay particular attention to the roof and wheel wells, and of course, take the extra time to clean of the windows and scrape the frost if necessary. (Or put a piece of cardboard across the windshield at night to avoid the frost altogether.)