Try to avoid the habit of keeping your foot on the brake, even lightly, when you are driving. When you rest your foot on the brake, you use more gas than you would otherwise and you also wear out your brakes much sooner.
If you find that you are keeping your foot on the brake without being aware of it, you can try sticking a reminder note to yourself on the dashboard of your car.
Start paying attention to your driving habits to see how important they are and to see how efficiently you drive.