Unless you're an electrical engineer, the only "common person" mods I'm aware of are tire pressure, unnecessary weight in the car (you could get lighter wheels than the stock wheels in most cases - I plan to do this in the next 2-3 months on my HCH), use the right kind of oil (0-20W for me). There have been discussions of the effect of various air filters, but I'm not sure if anything has been conclusive. Aside from that - driving style. I can claim an extra 6-7 MPG now above EPA due to tire pressure (45 psi) and driving style. Last four tanks have been above 50.

I've read about the "250 MPG" Prius, but the guy that did it is also an electrical engineer. There's enough volts in there to cause you serious harm or death - not something I'd ever play with unless I was an expert.

*** Retired after 65,000 outstanding miles ***