Try resetting the ignition timing with an electronic stobe light. Just one or two degrees too much retarded will reduce power and thus fuel economy. Try changing the spark plugs to Honda specification. Try adding 2 lbs of tire pressure to all four tires, over and above what the under hood decal says. Try using a thinner weight of engine oil, like 5W-30. If you have mechanical valves, adjust the valve lash exactly to specification. Has your car been in to any kind of service facility since you were getting better gas mileage.