there are about eleventy billion threads on this one....read the one about "Acetone"
you could do any number of things to improve your milage...
Tune up
New O2 sensors
Injector cleaner
Engine flush/change all fluids (TC, tranny, diffs)
there are a ton of little things you could do to to help by .5-1 MPG...or more.
BUT...for any real help, you need to be a lot more specific....how bad is your mileage...any other symptoms, running rough, etc...