Operate as small a car as possible for your driving needs. (Small cars weighing half as much as large cars use about half as much gasoline!)
Avoid cars with gas-consuming options such as air conditioning; power equipment such as window, door locks, etc.; automatic transmission, etc.
In hot climates, drive a car with light colored exterior and interior, to reflect light, heat. Tinted glass also prevents heat buildup.
Use radial tires for less friction between tire and road.
When driving, keep your eyes moving and your feet still!
Keep your steering wheel still too. The more you weave back and forth, the farther your car has to travel and the more gas is consumed.
Don't overfill your gas tank. It could leak or spill in heat or on a hill.
Use snow tires and/or chains as little as necessary because they make your car work harder and use more gasoline.
When you see a hill ahead, build up speed before you reach it, then maintain your speed on the slope. (If you must accelerate on the hill, you will use much more fuel). Then coast down the other side.
Keep wheels aligned for better mileage. Longer tire life, too.
Record all gas purchases for tax deduction purposes.