[2000mustang fan]

I bought my car about 3 months ago it's in excellent condiotion and only now has 65,000 miles on it. It's a 3.8L V6 OHV engine, About everything is stock except for the bodykit the car has on it that lowers to the ground and only one engine mod I did myself which was a cold air intake. My question is what do I do to improve gas mileage, remeber I can't sell the car I owe about $4k on it yet. and my car gets anywhere from 20-21MPG, this has only been achieved through a very small amount of hyper-miling and about half of passive driving techniques. I was thing about rear wheel skirts would that help at all and what materials do I use and ways to put it on that won't cause rust. What type of material can I use to fill in the grille with. Should i take off the stock wing or will that not make a difference. What do I do? I ordered a Tornado fuel saver, but it has not arrived yet. I also ordered some ethos, anyone have good experience with any of these. Thanks guys.