I found this neat article of a Volkswagen Rabbit that was modded to achieve 70 mpg. A lot of what they did is what we already know to do (belly pans, smooth wheels, grill covers, etc) plus a lot more complicated stuff like getting a diesel engine, clutch kit, etc. Many pictures and neat ideas that I haven't seen before so check it out!
The Rabbit Sipster
Ideas they mentioned not mentioned in the 65+mods:
greasing caliper pins on the brakes
adjusting rear drum breaks (if able to) to be slightly loose
Redline oil has gear oil and LSD friction modifier
aero front bumper to deflect air off of the side of the tire (pushing out vs. down)
aero hood vent (creates low pressure system above the engine to suck hot air out)"Basically, we cut two H-shaped slits in the hood, grabbed onto the sheet metal with our bare fingers, and bent the back of the steel down into the engine bay, and the front of it up into a little Gurney flap that should ensure a low-pressure area right over the opening to vent engine heat."