This is a small step that won't really make a huge difference at first, but over time, it'll be saving you gas and money. Little steps. Park and go inside of Taco Bell, instead of sitting in line, waiting for the drive-through. Turn your car off, when you're waiting in the driveway for your friend. If you're letting your car warm up, make sure you only wait a few minutes and don't forget that your car is out there using gas. Simple, but effective.
Some brands (like Mercedes-Benz, for example) are even starting to incorporate built-in start-stop "ECO" modes on their cars that do exactly this. They stop the engine when you brake and come to a stop at a light or sign, and start back up when you let your foot off the brake.