You can improve your gas mileage by changing your driving habits
Most of these suggestions will not only reduce the amount of gas you use but also reduce the wear and tear on your car and increase it's lifetime. More suggestions can be found at EPA's web site http://www.epa.gov/fueleconomy/whatyoucando.htm.
These suggestions when used in any kind of vehicle are likely to reduce the amount of fuel burned through normal operation.
Pump tires to recommended pressure
Having the tires at the right pressure decreases the friction between the road and the tires and reduces the amount of energy required for moving the wheels.
Go easy on the brakes and gas
Most of the energy of driving is spent on getting the car to cruising speed. If you need to stop at a stop sign it is a waste of gas to accelerate to 35 MPH just to slam on the brakes in 200 ft. Observe traffic and your surroundings and you will find many ways to avoid excessive use of the gas and brake pedals. Keeping constant load on your gas pedal also improves your gas mileage.
Observe the speed limit
At speeds over 55-60 MPH the resistance of your car increases drastically. This means that more energy is used to keep a cruising speed of 70 MPH than is used to keep a cruising speed of 60 MPH.
Get rid of the junk in the trunk
Carrying around excess weight increases the amount of energy used to propel your car.
Avoid idling
When you are idling you are getting 0 MPG.
Use fuel injector cleaner regularly
I find that using fuel injector cleaner with each oil change keeps my gas mileage high.