The only way to improve MPG is to keep your tires inflated, your windows up and your A/C off. Any aftermarket product that claims to improve MPG is probably junk. Properly maintaing your vehicle will do more than anything. We have had countless MPG threads if you don't know which maintenance I speak of, try a search. Keeping the EGR valve clean, using seafoam to clean carbon deposits, and utilizing no throttle coasting above 1100 RPM (when no fuel is being injected into the cylinders) will help give you a boost.

If you don't have any money to spend on gas, why not sell your car and buy something more economical?