If you're looking to improve your fuel mileage the best thing you can do is a proper tune-up. Replace plugs, wires, cap & rotor, fuel filter & air filter, set the ignition timing, clean your MAF, and make sure your tires are correctly inflated. Other correctable things that can affect your mileage would be incorrect wheel alignment, carbon build-up in the intake and combustion chambers, using dino oil that's too high of viscosity for your driving conditions, and carrying extra weight in the vehicle. If everything listed above is in good shape then you're probably getting as good of mileage as your vehicle is going to get.