Air up the tires to MAX psi, use synthetic in the WHOLE powertrain and use the best fuel you can. I perfer QT. 87 octane gets 1 to 1.5mpg better than cheap Conoco 89 Octane that's the same price. Also tune it up. Change plugs, wires, distributor (if there is one), air and fuel filter. Also check the O2 sensor, alot of guys on here highly recommend changing them. Outside of that check the other forums for mods you're interested in. Just depends on the time, money and dignity you're willing to sacrifice for better mileage.
120K do an Auto-Rx cleaning on the engine to start. As previous advice suggested I would run do a fluid exchange in transmission and fill with Mobil One or Redline. Also run a good oil and you can get away with extended oil changes.
Auto-Rx is a metal cleaner not a flush. Most people are getting a bump of 1 to 3 mpg after a cleaning of the engine. A internally clean engine is an efficient engine. Also run a good fuel system cleaner once or twice a year to keep it clean along with the injectors.