Your car has 96K miles an no known maintenance history. Time to baseline the car.
1) Fresh oil change
2) Fresh oil filter
3) New Air filter
4) Tire pressures? make sure they are up to snuff
5) Might consider MAF/O2 sensor changes/cleanings/replacements
6) You don't mention driving style. My non-S got good highway mileage.. maybe 25-28 but man, it was really bad on the track or AXs
.. How you drive will greatly impact mileage returned.
7) Might try some additives to clean out fuel injectors
8) Gasoline... Many folks experience a drop in fuel mileage in the winter time due to oxygenated fuels. It seems in the NE and Mid-Adlantic, MTBE (an oxyginator) has been replaced with ETHANOL blends.. up to 10-15% ethanol added. ETHANOL doesn't have as much energy as gasoline and therefore
hurts your mileage. Many states require posting of ETHANOL blends in gasoline so check the pump next time.
9) Catalytic converters get cogged over time as well... something to consider I guess.
Beyond that? not sure what else to suggest.