Use the good quality German-made parts for the important things like pistons, rings, head gaskets...
For more economy-
Reduce the weight of the car as much as possible, remove the spare tire if you want and consider removing the rear seat, possibly replacing it with a homemade plywood shelf to utilize the space.
Don't use a roof rack.
Reduce the amount of rolling mass as much as possible with smaller, light weight tires and stamped steel rims.
Lowering it might help, the underside of the car is not smooth and significantly contributes to air resistance, keep the front spoiler on to reduce the air flow under there.
Really though, suppose a difference of 10% in fuel economy takes you from 40 to 36 mpg, and say you have a long distance commute and drive 300 miles a week, you are looking at a difference of less than 1 gallon of fuel per week, it's really not much of a difference.