Hi,
Check out the aerodynamic tweaks on the Audi A4 that they just set the top speed record for a biogas powered vehicle:
(click on image for link)
Much of the upper part of the grill is covered (with acrylic), there is an exhaust louver just behind the radiator on the hood, the wheel covers are smooth (with printed on alloy wheel images!), the mirrors look smaller than stock, there is a wiper cover, the lower outer corners are smooth, and I'll bet the underpan is pretty darn smooth, too!