Summarize:

The AC uses gasoline to cool your car, which means you have less gasoline to actually drive with. Turn down the AC or turn it off once the car is cool to make your car more efficient. Running AC on max can decrease your fuel economy by almost 25%. The faster you drive your car the more fuel you will consume. This is not a small change, either -- every 5 miles per hour your drive over 50mph is the equivalent of paying $0.19 more for each gallon of gas. It takes more energy to start moving a car then to keep it moving. That means if you are constantly tailgating people, stopping and starting, or trying to pass, you're using far more fuel than if you were keeping an even pace. Try not to brake or accelerate harshly. Brake early instead of slamming on the pedal. Cruise control will keep your car at a consistent and even speed, which avoids burning up fuel unnecessarily with small accelerations and stops. Idling, or leaving your car on when it isn't moving, wastes gas without moving you anywhere. When possible, cut the engine to save on precious gasoline. These greatly diminish your cars aerodynamics, slowing your car down and causing you to use more fuel. In general, towing trailers or loading up the trunk is a more fuel efficient option. Under-inflated tires can lower gas mileage by 0.3% if all four tires are low. Using the free air pump at most gas stations, inflate the tires up to the PSI recommended in your owner's manual. Some cars list the proper tire pressure on a sticker in the driver's side door or the glove compartment. This is one of the cheapest and easiest ways to bump up your fuel efficiency. Make sure you buy the right filter for your car by bringing the make, model, and year in to your local auto shop and asking for recommendations--every car needs a different filter. For newer cars, replacing the air filter will not help fuel efficiency very much. It will, however, make it easier for your car to accelerate without issues.
Avoid using the air conditioner. Drive at the speed limit. Drive defensively. Use cruise-control for long, flat stretches. Turn off your car in traffic. Avoid rooftop cargo carriers. Keep your tires well-inflated. Replace your air filter.