Make sure you change your oil, your oil filter and your air filter on a regular basis. This will help your car to run more efficiently.
Don't speed. If you drive the speed limit, you will use less fuel. You will get to where your going on time, and you will save gas. Plan your errands. Try to get all your chores done at the same time, this will eliminate wasted trips and wasted gas use.
Don't lug a lot of stuff around in your trunk. The heavier your car is, the more gas you will be using, so now you have an excuse to clean all that junk you have been piling in for who knows how long.
It is amazing how a little planning and maintenance can save you fuel, but try it, and you will see it really does work.