This video is a good place to start. PriusChat is a good site with a wealth of Prius info.
I just got my 2005 Prius a few weeks ago. My only mod is 51psi (sidewall max) in the tires and a partial lower grille block that I removed last week after rain ruined the cardboard. I got 57MPG on my first tank. My driving technique involves mainly accelerating briskly. I try to keep out of the battery assist while accelerating, zero battery charge/discharge or battery charging are what I shoot for. Then I pulse and glide as much as I can based on lights and traffic. Being able to anticipate the lights in your area can be a big help. For braking, I try to avoid if possible, if not, I try to maximize the regen and minimize the friction braking. Which leads me to the gauges.
I'm currently using a bluetooth scanner and my old Android phone running the Torque app in my Prius. I like to watch my coolant temps (especially when blocking the grille), battery amps, friction brake torque, and ICE RPM because there isn't an obvious sign that it is off in the Gen 2. See
this thread on PriusChat for more on that. The Ultragauge won't show a whole lot more than the stock instrumentation. I would use it to watch coolant temps to see if its fully warmed up and able to do EV mode, but I think the Gen 3s are more flexible.
PriusChat has info about turning them into PHEVs. In my opinion, it probably won't save you any money unless you can drive it a
lot while in EV mode.