Over the last 6 mo, my gas mileage has been slowly dropping (27 to 23 mpg).
I adjusted valves to spec, ignition timing = 15 BTDC, verified no dragging brakes, new NGK plugs cap and rotor, tire pressure @ 32 psi, clean air filter, fluids to specs, spark plug wire 12K ohms or less, no engine missing or abnormal spark plug burn deposits, fluids OK and no CEL error light on dash.

Car has 160,000 miles on original OEM parts.
Summer of 2006, Honda replaced starter and did camshaft belt, water pump, oil seals, and replaced starter that failed.

Any ideas ? Emission controls like O2 sensor?

Part of the drop in MPG is a function of cooler weather; therefore, switch to synthetic engine oil and synthetic transmission fluid to help offset part of the drop. Another major contributing factor is likely a worn out oxygen sensor. If you want to go even further, replace your igniter and coil. In my opinion, these should have been changed at 120,000 miles or the 10 year mark. If you hooked a scope up to the engine, you would likely observe irregular ignition output wave forms developing. A final thought is to add Shell Regane or Chevron Techron to the gas, to ensure the injectors are clean.

Fuel filter