Reduce the weight of the car where possible. If you have a full size spare wheel replace it with a space saver or tyre mobility kit, use a lighter car battery and also make the wheels lighter too (forged). If you do these three things you will reduce the weight of the car down by about 40kg - 60kg (90 - 130 lbs).Also what would be the best upgrade to increase MPG.......
Also only drive around with your petrol tank half full. Not sure how much of an MPG difference you will see but its best thing I can think of.
Just one other thing, if your worried about MPG in this car, wait till the S5 comes along
Or you could save your money and mod the S5 when it comes, which is probably what I would do.