Try just doing the passenger side mirror, I did it, and my testing today showed a 3.7% gain (1.6 MPG). After that, look into smooth wheel covers, they are cheap fast to do and got me a 3.4% gain (1.4 MPG).
http://ecomodder.com/forum/showthrea...aps-14295.html
http://ecomodder.com/forum/showthrea...rix-14253.html
Also take a look at the Wiki page for car mods, it might spark some ideas:
Car Modifications Main - EcoModder