Highway pump the tires up 4 to 5 psi over the factory recommendation. Fill transmission and differential with a good synthetic. Look at installing a front spoiler to cut down the amount of air flowing under the van.
If your not using the rear seats unbolt them and put them in the garage.
Look for any other weight savings you can find and get it out of the van.
Don't destroy the van doing the weight reduction just the stuff you can bolt back on when you decide to sell it.
If your doing any highway driving keep it around 50-55 mph for maximum gas mileage.
ya i second the seat thing, then its like a pickup with a topper, when ya need to cram somehting big in it it just goes right in my dad did that to his aerostar, we never use the 3rd row rear seat so he popped it out and its sitting in the garage. easily removed prolly 70lbs...(huge bench seat) take out both theres over 100 and gained prolly 6ft of cargo room