You can buy a sports car, or:

Remember the tail fins on circa 1950 vehicles? They were beautiful but not very aerodynamic. At highway speeds, about 50% of engine power goes to overcoming aerodynamic drag, according to Consumer Reports. Wind resistance can reduce mileage, so you can maximize your mileage by removing luggage racks, roof-top carriers, and ski racks when they are not needed. Experts at Edmunds.com say that even keeping your car washed and waxed improves aerodynamics.

Source: EcoDriving USA

Also, remember that driving with your windows down or sunroof open also increases drag.