[Ed_T]

All good advice posted above. To add a datapoint, at ~sea level I'm getting
41.+ mpg using techniques to minimize gas engine use. Accelerating on downhills helps immensely versus accelerating uphill or on the flats. If done with a careful foot the mileage needle stays at the upper end. Slowly letting speed decrease on uphills helps also.

Hyper-milers use a pulse-and-glide technique. Cruise control on the highway is very important as previously stated. Steady speed is your friend with the caveats above. Foot off the gas well in advance of a slowdown or stop is a major plus as well as Jer stated.