Cold Air Intake/High Performance Air Filter
A better air filter and intake make it easier for your engine to breath, and can increase you
Mileage another 5% or so. It really depends on the car; some cars have very free flowing intakes, some have an extensive system of baffles and poor air filters. Replacement free flowing air Filters can cost as little as $20 online and are defiantly worth the money for most.Free Flowing Exhaust
Free flowing exhaust is a change that, while it can increase mileage, is usually more trouble
than it's worth. If your engine is breathing in more easily, you also want it to breathe out more
easily. The trouble is, exhaust systems cost hundreds of dollars plus the installation costs, and unless chosen wisely can hurt mileage as much as they can help.Decrease Drag
If your car has a roof rack you don't use, or a radio antenna you could do without, or a rear hood ordainment you don't particularly car about, get rid of them. If you want to get a little more serious, you can use foam, Plexiglas, tape or the like to smooth over places on your car that are not aerodynamic. If you own a truck, a truck bed cover or a rear cab can also very much increase your mileage. On some cars (research this first) a rear spoiler can help aerodynamics, though commercially sold spoilers are not designed to do this.
If you really want to go crazy, you can follow this guy's lead, and get 100 miles per gallon. http://www.autobloggreen.com/2007/12/27/aerocivics-mods-combine-great-aerodynamic-and-a-um-stunning-l/