How to improve gas mileage on my v-8 car if possible?

i have a 1985 mercury cougar, it has th 302 5 litre v-8...im takeing a trip to nebraska (from ohio) and i want to improve the gas mileage as much as i can, ive replaced the opil, air and crankcase filter....spark plugs are next, any other ideas like fuel additives or something?