ive seen great shops use the wrong oil, fill too much, or forget to change the filter. honda dealers are even worse. in some places they just drive your car around the block and park it and say youre done. here are some common things that would cause bad mileage that you can check.
-worn oxygen sensors (even tho they last upto 100k miles, they start to lose efficiency in as little as 20k miles and continue to get worse over time).
-unevenly worn tires or underinflated tires (honda recommends 30psi)
-alignment is out of spec (how straight the tires point)
-driving style (if you mash the gas or go above 3k rpms alot or drive in a hilly area, mileage will suffer)
-environment (gas mileage is much worse at sea level due to more oxygen, and gas mileage is much worse in cold weather for the same reason). more oxygen = more fuel dumped in by the ecu.
-loose gas cap or faulty evap system can cause much of your fuel to evaporate
-higher than normal idle can waste gas (should be around 700rpms)
-worn spark plugs wires and incorrect ignition timing
-sludge and deposits in the engine (try a treatment of auto-rx.com in the oil)
-clogged fuel injectors can cause a poor spray pattern (more of a squirt instead of vapor) which wastes alot of gas. try lubecontrol.com gas additive
-certain tires are more aggressive than others and can have higher rolling resistance, causing worse mileage. some hybrid vehicles get a 10mpg boost just by using low rolling resistance tires that come stock.
it sounds to me like maybe the engine is just getting old and needs some cleaning to work more efficiently. i bet with the auto-rx and then the lubecontrol products in oil and gas, your mileage will go up significantly. even on my relatively new and clean motor, i got an extra 4mpg on the highway using the additives. i have gotten 15 mpg in my 2002 civic before just based on driving style so i know its possible to go that low.