Plan ahead. Condense your errands so that you aren't running all around town in a disorderly fashion, using more gas than necessary. And if you are at a large shopping center where you need to go to multiple stores, park in the middle so you don't have to move your car for each store you need to go to.
When accelerating while driving, do so slowly. Putting the pedal to the floor uses more gas and is more inefficient. Try and keep the RPMs under 3000 for the most efficient driving.
Turn off your car when you aren't moving. According to one source, "An idling engine produces 50 to 100 more emissions than a warm engine. Plus, cars get the worst gas mileage when idling cold or warm. Gas is being burned for no forward motion."