Also, keeping your engine tuned and good operating order will help matters. Using a good oil will also help prevent friction issues which can affect the overall gas mileage. During the summer, roll the windows down which can also be helpful (no AC).
Also, have you considered buying your gasoline during the coolest part of the day? The gas is more dense during this time and releases less vapors. Many people do not think about this one. Every little bit helps you in the long run.