If you're on a regular street with traffic lights, be conscious about when you're accelerating and braking. Don't accelarate all the way up to a red light and then slam on the brakes. If you see a red light ahead, take your foot off the gas and coast up to the light. You can always accelerate again if the light turns green or you start going too slow.
Take all of the heavy objects out of your car. For example, if you carry golf clubs, a trailer hitch and a bunch of other heavy items in your trunk, then your car will have to use more gas to transport all of those items.
Make sure that your tires have the corerct amount of pressure. Deflated tires make it harder for your car to run.