Aerodynamics are important to a car. It's hard to make a car better aerodynamically - but it's very easy to make it worse.
*Blocking your grille has proven to be effective - just remember that your radiator (and grille opening) were designed for the worst case cooling scenario. So monitor your engine temps and adjust your grille opening accordingly.
- Fold your mirrors. If you can, fold your rear view mirrors in. Just be sure to stay in compliance with local laws.
- Belly Pan - A significant amount of resistance comes from the open areas under your car. Covering them with corroplast or some other material can reduce your drag coefficient (Cd)
- Wheel skirts - covering your wheel wells to keep your cars body one contiguous object can help
- Air Dam - see image, if you car doesn't have one already, this can help
- Always Drive with your windows up
The best thing you can do to help your aero resistance is to slow down. Resistance goes up with the cube of your velocity (holding everything else equal).