Precisely this is being discussed more specifically, with more specific answers, in the Prius forum:
http://www.greenhybrid.com/discuss/s...ad.php?p=99317
To generalize what we've found so far... using the battery is the #1 thing to avoid to achieve very high mileage. The cost of generating this electricity is too expensive - gas engine turning to fill battery to then dump back out to drive the wheels - all those conversions result in very low efficiency.
You do want to push the ICE at low RPM (the linked PDF is right about that), but not to the point that the car kicks in Battery Assist (I doubt the author realized that). On the Prius this appears as a yellow arrow coming from the battery. On the Honda Civic Hybrid this appears as the Assist bar growing - your goal should be to avoid the Assist bar when accellerating.
So far in the Prius this strategy has moved me from a max of ~55mpg with careful driving to at least 60mpg - and my mpg is still going up.