If you have an automatic, use the overdrive gear as much as possible. The octane level you use may also be wasting your hard-earned cash. Read your owners manual to find out what your car really needs. The next thing you should check is your tire air pressure and wheel alignment. Furthermore, this will wear out your tires faster, adding more cost to you.
Next thing is to get a tune-up.
Also, use your air-conditioning sparingly, as it is also a drain on your tank. Excess weight will also use more of your gas. While a few pounds may not make much difference every time you fill up, it can be a factor in the long run. Dump it in the river as soon as possible. Vehicles equipped with all-wheel drive or 4x4 options will also hurt you at the pumps.