Buy a Chevette. :wacko:
Actually, start with the simple things. Check your tire pressures to ensure they are at proper operating level, and i don't know if your rig is lifted or not, but, it may be cheaper to run smaller rubber, turning big tires eats alot of fuel.
Clean your fuel injectors. STP, Penzoil and a billion other companies make fuel injector cleaners. If they are operating right they will use a minimal amount of gas.
Check your filters. Fuel filters, Air Filters and Oil Filters. Try a tune up, if it hasn't been done in a while. Drive it gentle. I have a '93 Eddie Bauer that is lifted, has 32 inch tires on it, and just my style of driving can affect my fuel consumption. It has a 5.8 liter 351. If i hammer it to the floor all the time, i can almost watch the gas needle go down.
Other than that, you're correct, these things weren't built for fuel economy.