Gas prices are more expensive today then ever before. By driving sensibly you can save on the amount of fuel your vehicle needs to run, which in turn equals extra money in your pocket.
Avoid aggressive driving, rapid acceleration and hard braking because you are just wasting gas. Quick starts will consume huge amounts of gas because the engine has to work much harder to get the car up to speed.
Also, observing the speed limit can also help with fuel efficiency. While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.