See this thread, it's exactly the same discussion about the same vehicle:
http://www.gassavers.org/showthread.php?t=13712
In short: Inflate your tires to maximum, go medium-speed through green lights instead of stopping at red lights, brake less, coast more. Also, if you're willing to go slower on the highway, every mph you slow down will help.
Which rear end gear ratio do you have? Look at the label in your glovebox for RPO codes beginning with G. GT4: 3.73. GT5: 4.10. GU6: 3.42.