Muzzle Your Guzzle

Get in the habit of calculating your gas mileage each time you fill up

Just seeing what sort of mileage you are getting is the first step towards driving more efficiently. See our gas mileage calculation guide in the FAQ to learn how to do this.
Some cars have a fuel mileage gauge built in, which can show you a real-time mileage figure, as well as an average. Sometimes these are not completely accurate, so it's a good idea try calculating your gas mileage manually for two or three fillups to get an idea of how accurate your gauge is. Once you have an idea of the gauge's tendencies, you can continue using it to monitor your mileage.

Try laying off the gas pedal -- no more burning rubber at green lights, and no more speeding. Most cars' fuel efficiency drops off very rapidly as you travel faster than 60 mph. A good rule of thumb to remember is this:
Every 5 mph you travel faster than 60 mph costs you about 5% of your fuel efficiency.
With gas prices where they are now, that could mean two or three more dollars more per fillup, just for an extra 5 mph.
Most modern cars get the peak fuel efficiency between 30mph and 55mph. Try to stay in this range as much as possible
Keep your car clean and free of junk. Extra weight in your car means more work for the engine, which means lower fuel efficiency. An extra 100 pounds in the car can mean up to 2% lower efficiency.
Use your cruise control on the highway. This will help your car keep a steady pace, and will usually save you some gas.
Don't idle if you can avoid it. If you are going to be sitting for more than a minute, turn off the engine. Just remember: your car is getting exactly 0 miles to the gallon when it's idling.
Get into the highest gear possible when cruising, especially if you've got an overdrive gear. This will save you some gas and some wear on your engine.

If you haven't lately, take your car in for a tune-up. A car that's running out of tune is probably a car that's running inefficiently. Pay special attention if the check engine light comes on. While this could be due to any number of problems, many of them contribute to bad fuel efficiency. A faulty oxygen sensor, for example, could reduce your fuel efficiency by 40%.
Replace air filter if necessary. Keep an eye on this. A clogged air filter can reduce your engine's efficiency by up to 10%.
Keep your tires inflated propery. Under inflated tires make your engine work harder. Inside the frame of the driver's side door, all cars have a chart that details the recommended air pressure in each tire. Pay attention to it. A cheap cigarette lighter-powered pump can be found for $15 or less, and could pay for itself within a year if it means you keep your tires inflated properly.

All those trips around town really add up over time. Try to combine as many errands into one trip as you can.
Avoid sitting in traffic, where you are idling more often than not. Perhaps this means going into work and hour earlier or later to avoid rush hour.
Consider carpooling. This might be the single simplest way to cut down on your gas useage!
If you are lucky enough to have a job that allows you to telecommute... well, you get the idea.
Of course, use public transit systems if it's feasable.