Get your car a tune-up…that’s the quickest and easiest way. Make sure everything is in working order. Make sure your oil is good, and make sure things like your air, oil and fuel filters are clear of any garbage. Check your tire pressures, and fill them to recommended levels. Even though it’s minimal, try and save yourself from using your A/C too much. The effect of turning that off (there’s a clutch that engages, and takes power from the engine to turn the air compressor) might be negated by the lost aerodynamics of opening all of your windows though…I wonder if anyone ever studied that? You should also turn the A/C off before you turn yuor car off as well…it’s just one extra thing for the starter to spin when you start the car up again.
The best thing is to try not to be a lead foot. Don’t smash the gas petal at greeen lights, don’t speed like a mad-man, etc.
Finally…ignore anything that claims to give you better gas mileage, i.e. fuel treatment, high-flow air filters, that “Tornado” thing…if they worked, the manufacturers would’ve installed them themselves.