The Save the Flea project is an interesting read, as well as the
15 mods for 15% improvement.
The mods on Metro's Civic are pretty discreet and offer quite a bit of improvement over bone stock.
There are some other pretty simple mods that you can do that will help. From what I understand, an air dam and side skirts are some of the best aeromods since they basically don't allow air to get under the car.