Keep a small notebook in your glove compartment to write down the date and what was checked or replaced. Keep track of your gas mileage to make sure your car is not using more gas than it was designed to use. You may not notice a big drop immediately, or even in a few months. Check your tires to make sure they are properly filled. Low air pressure in your tires means your car must work harder to move along, and that eats up gas.
Tires lose pressure at different rates, so look at all the tires on a regular basis.
Be sure not to over-inflate your tires and make sure your tires are in good condition. Don't forget to have your wheels properly aligned as well. Change your fuel filter at least once a year, as it can get plugged with debris. This not only slows the process of gas getting to your engine, but it can stop your car cold if the filter is completely blocked and no gas gets through. A car-care professional will be able to tell you when you need a new fuel filter.