There are a lot of factors that can be causing poor MPG, but there is more information needed about the car itself and there is a plethora of information about "tune up's" out there on the good old inter webs already.
Anything from worn out injectors not firing correctly, worn plugs, tired distributor cap and rotor, air filter, worn out sensors can fall out of spec, tire pressure too low, even a worn out sticky brake caliper can just be slightly dragging and you would not even notice unless it was more drastic.
Any ways the list goes on you get it, the car simply needs to be combed over to see what is and what is not worn out and that will give you a idea of what parts you are going to ordered, other wise you are just guessing and that is a waste of money time and effort with out finding the solution possibly.