There are many factors which could influence your fuel consumption, namely, the design of the car itself - not much you can do about it. Are you getting what the EPA label said you would be getting?
Also, here are some things that will improve your fuel economy:
- Use full synthetic oil and change at regular 5k mile intervals
- Maintain correct tire pressure (look at the label inside the door jamb)
- If you haven't replaced your fuel filter yet since 2003 - time to do that now
- Buy some seafoam (or STP alternative) before you change your oil and add the full bottle to the crankcase and drive at least 50 miles with the seafoam in there - this will clean your engine
- After each oil change, add some seafoam (or alternative) to your gas to clean the fuel injectors.
- Check your alignment/tires. Bad alignment can cause your car to drag (also bad for the tires)
- And lastly - drive smoothly, don't accelerate quickly, avoid braking and speeding up. Take it easy.