It might sound obvious, but don't push the gas pedal as much.
For example, instead of flooring up a big hill on the highway to maintain your speed, accelerate slightly before the hill and let your momentum move you up. You'll end up sacrificing some speed, but you can re-accelerate later when the road has leveled out. (For the love of God please move over to the right.)
Ditto at stop signs, and traffic lights. There is no need to get up to speed in a split second. Smoother and slower accelerating will save you a lot more gas then you realize.
Same thing when driving in the city. Try to coast as much as possible behind the car infront of you. Avoid riding his bumper, forcing you to brake and accelerate constantly.