Check to make sure you're not carrying any extra weight or unneeded items in the trunk or backseat of your car.
Use cruise control (if you have it) on the highway.
Turn off the engine instead of idling when you plan to be waiting for more than 3 or 4 minutes.
Avoid jackrabbit starts and speeding from one stop sign to the next. Accelerate slowly.
Use only high-octane gasoline (the expensive one at the pump) if your engine is pinging. Pinging is a rattling-type noise from the engine, mostly heard on acceleration and when driving uphill.
Keep all the tires properly inflated. The recommended tire pressure is listed in the vehicle's manual, stamped on the side of the tire, and often on a sticker on the driver's-side doorjamb. When in doubt, 32 pounds per square inch (psi) is a good average until other sources can be consulted. Low air pressure will cause bad gas mileage.
Follow a maintenance schedule. Most cars need a tune-up every year or 30,000 miles, whichever comes first. Older cars need to be serviced more frequently. A car in need of a tune-up may exhibit poor gas mileage as a symptom.
Change the air filter every 15,000 miles.
Drive just 55 mph on the highway if traffic allows you to do this safely. The faster you drive, the more gas your car will use.
Shut off the air conditioner when you don't absolutely need it.
Keep the clutch properly adjusted. A slipping clutch uses extra fuel.