Best would be closely following a friend in a truck.
I was on I-5 once and a 18-wheeler pulled slowly past, and pulled over right in front of me. It had a half-height full width load which seems ideal as there is less buffeting. So I stayed in his blind spot for 80 miles.
It wasn't documented, but as basjoos says, it probably about doubled my mileage. But I picked up a lot of rock chips in the paint and I was a nervous wreck, so I don't do it anymore.
Be aware that the boattail that MetroMPG posted has a bad mismatch at the trailing edge of the hatch. The whole top or at least 50% will be in turbulent flow.
Instead of rear wheel spats, maybe something like this:
Instead of a tab running across the tread, divergent fences that force the wheelwell turbulence outward where it can be sheared off by the ambient flow, instead of under the car. Adjusted for adequate departure angle of course.