Mileage decreases sharply above 55 mph. Use cruise-control on open highways to keep your speed as steady as possible. Avoid excessive idling. Shut off the engine while waiting for friends and family. Remove excess items from the vehicle.
Less weight means better mileage.
Consolidate your daily errands to eliminate unnecessary driving. Join a car pool. Remember, how your car runs, how you drive it, and how its fluids, old parts, and tires are disposed of all have serious consequences on the environment.