Keep the spark plugs in good condition. Some last 100,000 miles, but many need to be replaced more often. Check the air filter twice a year, more often in dusty conditions. Check the air pressure in your tires. Inflate according to the owner's manual.
Underinflated tires can cut gas mileage.
Replace parts that reduce your engine's efficiency. For example, a defective radiator thermostat can make your car run too hot or too cold.