Mind your maintenance. A well-tuned car burns less gasoline, so get your oil and air filters changed regularly. Not sure if your filter needs changing? Take it out and hold it up to the light. If you can't see any light coming through, it's too dirty. It's also important to check your tire pressure monthly. (You'll get the most consistent results by using your own gauge when the car is cold.) Americans driving on under-inflated tires waste 4 million gallons of gas a day and reduce the lifespan of their tires.
Clear out the clutter. According to the Department of Energy, every 100 pounds you carry inside your car lowers its fuel economy by one to two percent. So if you don't need it on the trip you’re taking, don't have it in your trunk.
Don't drive like a jerk. Fast, aggressive driving is a big fuel waster. Try to maintain a constant speed--most cars reach peak efficiency while cruising steady between 55 and 60 miles per hour--and avoid rapid acceleration and braking, which can increase fuel consumption by as much as 40 percent. (And make everyone else on the road hate you.)
Keep your cool. Need relief from the heat? Consumer Reports recommends using the air conditioner if you're going faster than around 40 miles per hour, since a lot of engine power at high speeds goes to reducing drag. When you're cruising around town, it's more efficient to keep your windows open.
Think different. Could some of your short trips be made on public transportation, or by walking or riding a bike? Combining trips also helps, as does carpooling with friends.
Want to know how much you'd save on gas if your car got 40 miles to the gallon? Visit the Sierra Club's MPG calculator. Just enter in your car's make, model, and how many miles you drive each year, and it'll do the math for you.