Get a new girlfriend
60 is beyond incredible in that car. Don't change a single thing or you risk taking it back to 35.
Make sure your tires are properly inflated, use cheap gas, don't increase your compression (since that will change BSFC for the better but possibly MPG for the worse in that car since its already operating at a lean limit) Altering the exhaust in that car can also drop MPG since you are at the lean limit to start with, and that car relies heavily on exhaust scavenging for its low RPM torque. Many times a performance exhaust will kill torque but add a couple HP. In order to compensate you'll have to bury the throttle to keep up on the highway which will kill MPG.
Normally, BeefBourito would be spot on, but these economy cars are already at certain limits and modifying them sometimes just makes things worse.
If you're getting 60... holy crap man. Just drive. I get 9 mpgs in town. 9!!! I just drove from California to Pennsylvania, so I'm feeling the sting.