If you have already tried driving at a steady speed, avoiding sudden stops and decreasing your top speed to increase your fuel economy without much luck, then there are some other factors that you may want to consider.
Take a look to see if you can remove some weight from the vehicle. Hauling around extra luggage causes the vehicle to have to work harder decreasing your fuel economy. This is generally most effective in smaller vehicles.
Grab your owner’s manual to see what type of oil gets recommended for your vehicle. Generally, synthetic oils lubricate the car better...