Go look through the wiki:
Car MPG Efficiency Modifications Main - EcoModder
There are lots of mods and user data like MPG gain, time to do the mod, cost and how to's.
I have done that same trip (MI to SC) several times and I found that a kill switch was nice for the mountains. Smooth hub caps are also nice.
You could take off your side mirror and put one on the inside. Most cars only have 3 bolts holding the mirror on. its a quick 1.6 MPG on my car.