There are 4 reasons for poor gas mileage.
1) Tune up needed
2) Carbon build up or dirty fuel system
3) Mechanical / emissions problem
4) Driving habits
The first thing I'd do is check for any vacuum leaks. Next I'd change the air filter if dirty and run some Techron as suggested or try Seafoam. Seafoam is fantastic for getting rid of the carbon build up and cleaning out the fuel system. If that doesn't fix the problem then do a complete tuneup (plugs, caps, rotors, wires, fuel filter, pcv valve). If you are still getting poor mileage then you may have some other problem. Has the check engine light gone on at all? If after doing all of the above and you still have poor mileage then..........SLOW DOWN! :D just kidding