Check the Tires
Tires with low air pressure mean that there is more contact between tire and road, which means more friction. Lower tires also create more "give," which means it takes more energy to start moving. Check your tires regularly to ensure they're filled to the manufacturer's recommended level. Also, if possible for your make of car, choose narrower tires, which mean less contact with road.
The larger and heavier your vehicle, the more energy you need to move the vehicle. If it's not possible to trade in a larger vehicle for a smaller one, you can control how much weight you carry around. Avoid storing heavy items in your vehicle. If something is not needed on a regular basis, remove it from your car. If you drive a truck and live in a cold climate, remove sandbags (used to create extra weight and traction in winter months) during warmer months. If you move or purchase a heavy object, take it out of the car as soon as possible. Remove anything you don't use on a daily basis, except for emergency items.
No, this isn't about the music you listen to. Keep your car tuned up for maximum performance. Have your spark plugs checked regularly to ensure they're tight fitting and clean, and that the wires are in good condition. Make sure the fuel injector is clean and operating properly. Check the exhaust system regularly to make sure it's clean and working (if your vehicle is running loudly, it may be past time to replace the exhaust). Also, check that your vehicle's timing is correct, and that all filters are unclogged.
Use the type of fuel recommended for your vehicle. While the sales pitch for higher-octane fuel is enticing, in many instances the price you pay for the fuel will not improve your car's performance or your gas mileage. Also, be wary of fuel additives that claim to increase your mileage. These products may not affect the performance as much as driving appropriately and keeping your car in good working condition. In other words, they're not a substitute for other methods that will have a positive effect on your mileage.