These are not costly, expensive or complicated measures such as changing the computer chips or fuel delivery systems in your vehicle. The keys are something as simple as reducing weight and friction. The greater than either, or both, of these two characteristics are in your vehicle, the more gasoline you will use. First let's look at the weight factor. Without taking a cutting torch to your vehicle, or drilling lightening holes in the body and chassis, you can reduce the weight.
Sprung weight is exactly what it sounds like - the weight born by the vehicle's suspension.
Also, the old adage that a clean car runs better has some basis in truth - not from any gains in aerodynamics of a clean body, but in less weight of the junk you are hauling around. Cleaning the chassis, motor and undercarriage with a pressure washer periodically will also help. Built up mud, tar and road grime on the undercarriage, motor and chassis add much more weight to the car than the little bit of dirt you wash off the body. However, even relatively large weight reductions in sprung weight will not give you the gas savings that a small reduction in un-sprung weight will. Un-sprung weight includes the weight of the axles, wheels and tires.