Turn off the engine if you anticipate a lengthy wait like when waiting at a train crossing for a train to pass. Instead of idling at a drive-up window, park the car and go in. Idling burns more gas than restarting the engine. Limit car warmups in winter. Don't carry unneeded items in the trunk .
Extra weight decreases gas mileage.
Also, reduce drag by placing items inside the car or trunk rather than on roof racks. In cold weather check the oil level in your engine . Cold weather starting does not allow the car engine to heat up quickly enough to seal the engine parts. Thus more oil is used. Avoid high speeds .