GET MOTORING.  After starting your car drive off as soon as possible. Sitting there idle wastes gas and results in unnecessary emissions. Plus it looks sort of weird.
SHED THE LEAD.  Harsh acceleration and heavy breaking increase fuel consumption dramatically. Don't drive with a lead foot. Think more along the lines of driving with a feather foot.
TAKE IT EASY.  Decreasing your speed can significantly increase your fuel efficiency. Every mile per hour faster you go over 55 mph, your fuel economy decreases by 1%. And even more than 1% over 65.
DITCH THE EXCESS BAGGAGE.  Don't carry unnecessary weight, and remove roof racks when you're not using them. An extra 100 pounds can reduce your miles-per-gallon fuel economy by 2%. Since this is relative to the weight of the car, this is especially true for smaller cars that weigh less. Whether or not your friends and loved ones qualify as excess baggage is entirely case dependent.
GET ACQUAINTED WITH YOUR CAR'S AVERAGE FUEL CONSUMPTION.  It'll help you recognize drastic fluctuations in overall consumption, which may indicate your car needs to be serviced. Regular service checks will ensure your car will run as efficiently as possible. And that will save you both kinds of green.
KNOW WHEN TO COOL IT.  Running air conditioning continuously burns a lot of fuel - it could decrease your car's fuel efficiency 10-20%. However, driving with the windows open at high speeds can also decrease fuel efficiency. The general rule is to keep the windows down while on city streets, driving under 40 mph or in stop-and-go traffic. But as your speed increases to 45 mph or highway speeds, switch to the A/C as driving with the windows down increases the drag, resulting in decreased fuel efficiency of up to 10%. Speeds over 55 mph with windows down can decrease fuel efficiency of up to 20% or more.
PUT IT ON AUTOPILOT.  Save fuel by using cruise control. This will lessen your need to accelerate, which burns fuel quickly, and also prevents you from exceeding the speed limit, which burns fuel at an even faster rate. It will also save you money by avoiding speeding tickets.
CLEAR THE AIR.  Check and replace air filters regularly. Keeping clean air filters prevents any restriction of airflow to the engine. Too much restriction of air causes gas-powered engines to work harder, resulting in more gas burned.
LEAVE IT TO THE PROS.  Taking your car to a commercial car wash can actually be better for the environment than washing at home. Several commercial car washes use recycled water, and on average can use less than half the water than washing at home. And it keeps you dry, as long as you keep the windows up. If you need to wash your car at home, use less water and bio-friendly soaps to help protect unnecessary chemicals flowing into the sewers.
BEAT MOTHER NATURE AT HER OWN GAME.  In the summer, park your car in the shade to stay cool and prevent gas evaporation. In the winter, use a windshield shade to minimize frost to avoid having to run the heat excessively during start up which burns excess fuel. In this case, side-stepping Mother Nature is for her own good. Literally.
CHOOSE YOUR OCTANE CAREFULLY.  It's not always true that one octane fits all. There is a specific octane that's right for your car. Certain engines reduce power when using regular octane, forcing the driver to use more throttle in order to maintain speed. With the proper octane your car will run more smoothly and efficiently. You'll be happy. Your car will be happy. And so will the planet.
GET YOUR WHEELS IN LINE.  Gas mileage increases as rolling resistance decreases. Total alignment sets all four wheels parallel which, along with proper inflation, minimizes rolling resistance. It increases your handling control as well, which gives you more control of your car and your impact on the world around you.
TAKE A DIP IN THE CARPOOL.  The best way to reduce emissions is to get more cars off the road. Even if you carpool once a week you'll be making a difference. Either drive a carpool yourself or set aside a "no driving" day each week to carpool with someone else. It may even get you to work faster as many cities have HOV (High Occupancy Vehicle) lanes reserved for carpoolers. And it's a great way to promote fellowship and camaraderie.