one of the biggest things you can do is go easy on the throttle. a vacuum gage helps you see how much youre giving it, so you can see the need to pull back a little and save a lot of gas.
regarding the K&N that bubba mentioned, those have been disputed back and forth, and the general consensus from what i've heard is they can't flow any more air without reducing the quality of filtration, and from what i've heard theyre not a good fit for an economy minded user
83 F250 flatbed 300-6, NP435, 4X4, 5800lbs empty weight
87 E-350 6.9 diesel, c6, 3.55s, powertrax no-slip locker, onboard 120v power and compressed air, built out with toolboxes and toys