You could upgrade the exhaust. You might get a slight bump in power, as well as a small increase in mileage. Also, you should make sure all maintenance is up to date, like plugs, wires, air and fuel filters, transmission fluid, engine oil, etc. Don't forget to make sure your tires are properly inflated too.
With all that said.. the best way to get max fuel economy is to adjust your driving style. There are so many small bad habits that reduce gas mileage. When I drive to my parents' (about 400km), when I don't use cruise control, I average about 25mpg..but when I use the cruise control, and take my time, I can get almost 35mpg.