Check out WikipediA articles on hypermilers and fuel efficientcy. Both of these have tips on how to better your fuel economy. From what I've heard about the Prius EV mod, it might not improve fuel efficeintcy since it burns lots of battery that the ICE must come on and replinish.

Dan is an official (Honda) Hydrogen Economy Convert!
Houston Hybrid and Hypermilers Club <106mpg.com>

Best commute = 14.3mi @ 114 MPG (sg2)
Best (non-trivial) tank = 769mi @ 85.7 MPG (fcd)
MPG Centurion-Hybridfest 2007-Prius II-26mi @ 106 MPG (sg2)
Dan <11011011> @ CleanMPG.com