I would suggest you start off with the simple things, to keep things cheap, easy and get you a little extra mileage while you are getting familiar with some hypermiling techniques.
Max sidewall in your tires: Free, 5 mins.
Antenna Delete/Relocate: Free, an hour or 2.
Mirror Delete/Interior Mirrors: Free-a few bucks, an hour tops.
Partial/Full Grill Block: Free-a few bucks, an hour - a few days depending on complexity.
Those small and quick upgrades should net you a few mpg and not take long at all to do. It would be a good start and since they are so cheap and easy, there are no excuses for procrastination (my biggest problem).
After you have done those, you can move on to the more complex and time consuming mods (belly pan, aero-cap, wheel skirts, air dam, etc.)