Re. O2 sensors: if you decide to replace an O2 sensor make sure you
replace all the O2 sensors on your car at the same time. I've had good
results with Denso O2 sensors in both Honda (190K miles) and Toyota
(170K miles) products. These are available with OEM connectors at about
half the price of what looks to be the exact same parts in a Honda or
Toyota parts box. New O2 sensors improved the mpg in both the cars I've
put them in.

As others have suggested, to increase mpg start with the basics; air
filter, distributor cap, rotor, new spark plugs (standard NGKs, nothing
fancy) and check for tire pressure. At 8-9 years your ignition wires
are getting a little old and replacement isn't a bad idea if you intend
to keep the car. Your O2 sensors are probably OK for another 50K miles.
Doesn't this engine require periodic valve adjustments? The timing
should have been set when the timing belt was replaced at 90K miles but
checking is a good idea.