There are many ways to save fuel while driving. The biggest and most obvious, as well as most convenient is to avoid traffic. Stop and go is the worst for mileage. So lights would be a good thing to avoid. In the summer time, especially during highway driving using the air conditioning can save versus rolling the windows down because of the aerodynamic drag involved.
Shifting to a higher gear and running your car at a lower engine speed will increase mileage as well to a certain extent. As mentioned by Chris_L keeping your car properly maintained is of utmost importance in both mileage and reliability.
Also not having accessories on the outside of your car will decrease drag and increase mileage. For instance if you want a roof rack look for a detachable one and use it only as needed.
Those would be your most likely ways to save fuel but you could try things like a performance air filter to improve the engines efficiency. It can provide a mile or 2 per gallon depending on the vehicle and does improve performance slightly.