Mileage or HP upgrades

ok, so i got a 93 cutlass supreme, 3.1L V6. i like this car, and i wanted to know if there were any upgrades that could boost my gas mileage, maybe even my power, but only when i get on it of course...as for upgrades im talking about everything...from the little things, to the bigger ones like headers and major parts. or maybe things like gauges where i can feather the throttle to keep it at a good efficient pace...any ideas would be great