How To Improve Fuel Economy In Impala?

I just boought a 2003 Chevrolet Impala (3400 Motor) recently, and of the main reasons it was bought was for the good fuel economy (EPA estimated 21 City, 25 Combined, 32 MPG City). Well the problem is it is only getting 18 MPG combined. The car has everything replaced in it recently, oil, transmission fluid, tires. My question is, what else would be causing this huge lack of good mileage?