Keep tires inflated to recommended pressure. That can be worth 1-2 mpg.

Keep engine free of carbon build up. a lot of professional mechanics add BG 44K to the gas tank. It not only maximizes economy, it can greatly reduce emissions and improve drivability a lot.

Have your oil changed as recommended by the manufacturer. Dirty oil and a clogged filter can cost you up to .5 mpg.

Make sure air filters are changed when needed. A dirty one can cost up to 2 mpg.

Keep car tuned up. Not only can an out of tune engine greatly increase emissions, but can drag your economy down by as mush as 10 mpg!

Drive gently. Hard acceleration decreases fuel economy.

Don't use the A/C if you don't need it. That'll gain you about 1-2 mpg.

Keep wheels properly aligned. If they're not, it can decrease economy by up to 20 percent, as well as increase tire wear.