65+ Vehicle modifications for better fuel economy - EcoModder.com
Most improvement will come from adjusting the nut behind the wheel. The difference between me and my wife driving our Insight is often in excess of 30mpg.
Some proven, easy, reversible mods for the car include a partial grille block (use election signs) and increase tire pressure (I run max sidewall). Several car manufacturers were recently caught "cheating" fuel economy tests by running slightly higher tire pressure.