The easiest thing that you can do to get better gas mileage is change your driving habits. Most important is drive the speed limit. Whenever you are driving over 40 mph your gas mileage is dramatically reduced because of the wind resistance. This factor increases the faster you go. Keep your speed steady as accelerating uses more gas.
Cruise control is a great way to save gas on the highway.
Plan your travel carefully to take the shortest, most direct route. Drive on highways when possible. Accelerate before you reach hills, not while you're on them. Traffic lights, stop signs and bumper to bumper traffic use more gas because of all the braking and accelerating. Most important is to complete all your errands in one trip, you will instantly lose your savings if you have to repeatedly go back to the store for one or two items.