I suck as a driver, so all of my gains are from the vehicle despite its owner!
If we had a Jedi Master hypermiling my car it would probably do pretty well.
Warm air intake
O2 sensor modified to run leaner
Intake air sensor modified to advance timing
Upgraded to high performance catalytic converters
Tires overinflated to 50 PSI
Titanium thin sheet metal lower grille block (a bear to make, but looks ok!)
Small weight reductions here and there
Improve the warm-air intake
Fabricate a fuel vaporizer
Rear fender skirts
2" drop w/performance springs
Weld racing aero wheel covers
Goodyear Fuel saver tires
Extra batteries for more storage
One small photovoltaic cell
More small weight reductions here and there to hopefully add up to something someday!
WISHFUL THINKING (Maybe next year):
Weld Wheels Draglite rims
Full air dam front bumper
Remove a few brushes from the motor for more output (I have no idea why this works, but our college electric car set a world record a couple years ago-- and they did this)
REALLY WISHFUL THINKING (Maybe in a few years):
Swap out the gas engine for a smaller diesel, but keep the hybrid stuff, and keep the vaporizer on there.