How to improve fuel mileage ?

I have a 1999 grand prix gt with 213k on it. The car has had regular maintenance through the years and the mpg has dropped the last 6-8 months from 28-29 to 24-25 tops. These are all hwy miles also. Car runs great; no sputtering or missing. Any recommendations to what I can do to get back the mpg I lost or is this common with the high miles?