As others have said, it's mostly the nut behind the wheel and not so much the machine. Good maintenance is important but doesn't offset "normal" (meaning - the usual unconscious, ill-advised, distracted, skill-free typical) technique.

I drive 100 miles/day commute to/from work ( karmic payback I guess for the 17 years where my commute was 2.5 miles). I've been able to get about 20% better mileage on a mostly interstate highway profile by adopting some additional techniques over my previous conservative and boring driving style. I have a mpg meter on the car that has helped me refine my techniques and rewards me with higher numbers if I do everything right and catch some lucky breaks on timing stoplights on the route.

Hypermiling has many valuable techniques in their list but they also have some that are uncomfortable, likely to cause more maintenance and up into outright dangerous behaviors.

I'm sure the construction guys in their empty F350 pickups I see blazing along tailgating and laneswitching their 90mph way to work are paying at the pump. They probably don't want to know how much of the bill is under their control. (Don't try to tell them either!)