Always drive downhill. Many downhill stretches can be traversed with the engine shut off entirely, for an effective fuel consumption of ∞ m.p.g. When you reach a low point on your projected route, sell your car, and buy another one at the top of the next hill.
Remove excess weight, such as luggage, groceries, spare tires, children, doors, bumpers, etc. The heaviest single object in your car is usually the engine, the removal of which can lead to a dramatic dip in your fuel consumption.
Streamline your car by filling in irregularities in its shape with modeling clay. Of all the irregularities in the outline of your car, the one that usually affects fuel economy the most is the deep notch formed by the the obtuse angle between the windshield and the hood.
Instead of gasoline, try oxen.
Inflate your tires with helium to lighten your vehicle by up to 0.000018%.
Drive as fast as possible, so that you arrive at your destination before you have a chance to use very much gasoline.
Fold an origami car, following one of the many popular patterns available on the Internet. Paper is much lighter than the steel and plastics used in the construction of mass-produced vehicles, and you will be driving an original work of art as well as saving fuel.
Instead of going on a driving vacation this summer, sit in your chair at work with your hands on an imaginary steering wheel and say “Vroom, vroom” every once in a while.