Bought this car used, in mint condition, in Feb. '07. Currently approaching 160k miles (Oct '08), and averaging 41+ MPG.
The HX model uses Honda's VTEC-E intake system, which goes for fuel economy what regular V-TEC does for horsepower. It forces the engine to operate at 2 valves per cylinder at low RPMs for better torque and lower fuel consumption, switching to 4 valves at ~3k RPMs best I can tell. Add Lean Burn into the mix, and MPG shoots up on the highway. Moreover, the 5-speed manual tranny is geared VERY tall, so 60 mph take about 2100 RPM.
While not as miserly as the old Civic VX or CRX HF, this car has loads of potential for hypermileage.
Oversized oil filter
Mobil 1 5w-30 (for road trips)
Tires up to 41 PSI
Scangauge II + GPS
Full upper grill block
80% lower grill block
Set of 44 PSI sidewall tires, pumped to 50 PSI
pizza-pan wheel covers (maybe)
0w-20 motor oil (if someone else tries it first!)