Adjust the nut behind the wheel... Sorry, couldn't help it. Honestly as long as the vehicle is in good condition, spark plugs/wires in good shape, regular oil/filter changes and air filter changes there is not much else that can be done besides driving it like a granny.
It is possible that adding an aftermarket intake, header and exhaust would help with mileage a little bit as it is making the engine run more efficiently, but if you start using that new found power you would be using more fuel than before.
