When I purchased my van it only got about 16 miles per gallon. So I set out to improve on that. First replace the spark plugs. Use platinum as a minimum, there is also split-fire and ring-of-fire plugs that will help give better MPG's.
Second add some magnets to the fuel line. Here are two sets of hard drive magnets on the fuel line of my 2001 Ford Windstar.
Next add a spacer to your oxygen sensor. The problem is that the car is running so well that the computer thinks it needs to pollute some more. Adding the Oxygen spacer tells the computer that the exhaust is more polluted so it will stop dumping so much fuel.