Drive softer. No fast starts, no aggressive driving, and slow down. That will often get you 5-10% better mileage right off the bat.

I'm a big efficiency person in that I dislike any sort of sloppy engineering system that wastes energy or material. But one thing to think about on the Prius is the added cost of the vehicle (~ $4000 USD), that it's batteries will eventually need to be completely replaced and that will cost thousands of dollars (and if it happens at say 125,000 miles, would you really want to sink that much $ into a vehicle with that sort of mileage? Something to think about), and I have also heard that it's little tires wear out faster than usual. I have never seen a real life, comprehensive accounting of what all this will cost w.r.t. what you are saving in gas........ It would be interesting to see.