What's a great way to increase your vehicle's mpg?

I have done a few things to my car (2003 Mitt. Lancer ES) that have increased my mpg. The first thing I did was to switch to a K&N air filter, then I bought the new E3 spark plugs and I switched from regular motor oil to synthetic(Royal Purple is my preferred brand). All of this increased my horsepower and mpg a bit, but what has worked the best on my car is using Z-Max. Within the first 30 minutes of driving, my car was much smoother and I have a lot more horsepower (takes about 1000 miles for the Z-Max to soak completely into the engine block and is then good for about 6k miles) and my car has gone from an average of 25 mpg(when I bought the car) to now averaging around 35/36 mpg. I use the Z-Max once every 6 months to maintain peak performance and fuel economy. The Z-Max usage easily pays back quick in the increased mpg that you get and fewer trips to the gas station....

Your engine gets the worst MPG (and the most engine wear) in the first few minutes of driving. So if you take several short trips per day, an hour or two between each, you can easily improve your MPG by making all the trips in one combined trip, where your engine gets warm and stays warm. Plan your trips to to create the most efficient route.

Shift as early as your transmission allows (if you have a manual tranny). Accelerate gently. Anticipate stop lights -- if you time them well, you never have to completely stop, which translates to better MPG, as well as longer brake and tranny life. Keeping engine load as low, and as constant as possible will make more difference in MPG than almost anything else you can do. Use cruise control, it keeps speed more constant than the human foot (unless you are in the hills).

Optimal MPG is in top gear between 45 and 60, hence the old "double nickel." But since the speed limit is a limit to your upper speed, you are allowed (if it is safe to do so) to drive slower. Say, 55. On a 60 mile trip, this makes less than 10 minutes difference time, but can save about 20% in fuel (compared to 70). Cruise control can help, here, too.

Remove excess "kit" from your car. Do you carry 5-gallons of water everywhere? How about a big toolbox? Take out the stuff that isn't needed for the current trip.

Remove rooftop carriers AND crossbars. Car & Driver did a test late last year, and found that the crossbars alone caused almost as much drag as the rooftop carrier did. If you are on the highway a lot, remove anything that adds drag, such as KC lights, traailer mirrors, etc.

Don't idle: idling = zero MPG. Besides, it isn't particularly good for your engine.

Add a cold air intake system. Raise the tire pressure. Get a fuel system cleaning. Don't use your AC. Work a few extra hours to make up for the gas. =)

In some of the WWII aircraft engines, the Merlin engine in particular, steam was used to up the compression ration, and this the power band.

Make sure your car is in proper tune and make sure your tires are properly inflated too. This will make more of a difference than you might think!

keep it clean - change the Airfilter drive carefully and stay within the speed limit