There are a number of ways to increase your gas mileage whether your car is new or has been around the block a few times.
Go easy on the accelerator. Do not take off to a quick start and drive as if you are a race car driver. This wastes gasoline and is dangerous anyway, as well as being hard on your engine.
Keep your car tuned up at all times. A badly tuned car will burn a lot of gas and cause pollution as well.
Remove excess weight from your vehicle. The heavier a car is, the more energy it takes to move it.
Inflate your tires properly. Under inflated tires wastes gas and can cause damage to the tires.
When you can, use cruise control. This always helps, in that on a long trip, a person is likely to let up and down on the accelerator, wasting gas and energy.