Get your car filled early. As the temperature increases, some of the fuel in the tanks takes the form of the fumes and is wasted. Early in the morning, the fuel is in its best liquid condition so none of it is wasted.
Drive at a Moderate Speed
If you maintain a high speed over longer periods of time or accelerate a lot, more fuel will be consumed. Maintain a moderate speeds with lesser braking requirements to save on fuel and thus money.
Smaller Engine Sizes
It is not a day and age to own gas guzzlers. Look for cars with smaller engines such as 1.5 to 2.0 liters so that lesser fuel is consumed. The cost of maintenance as well as gas expenditure will be drastically reduced in comparison with a car with an engine of 3.0 liter or higher.
Purchase a hybrid vehicle. It will cost more when they are purchased but it will save a lot in the longer run. Typically these cars offer a mileage of over 30 miles per gallon which is quite good.
Lesser Air Conditioning
Avoid air conditioning whenever possible. This can be done at daytime when the temperature is moderate and during the evenings/nights. Fresh air feels good too and it will also play a part not only in saving money but doing your bit for the environment.
Walk whenever it is possible. If you have to commute to a distance of a couple hundred meters or so, walk instead of taking the car. It will add to your health and save you quite a bit on the gas in the longer run as well.