Well aside from not actually driving, there is a very simple thing that can be done to maximize your fuel economy. A couple years ago, there was a study done to determine how much simple actions affected your gas mileage (lost the source sorry). The results were only a little surprising. The biggest factor was using moderate driving techniques. This means that you do NOT punch the accelerator when you leave a stop light or stop sign. And instead of gassing it all the way to the next stop and slamming on the breaks, you let up on the gas early and make a more gradual stop (your passengers will also thank you). This moderate style of driving can gain you upwards of 30% fuel economy if I remember correctly.

Other factors such as rolling windows down instead of using air conditioning only resulted in a negligible difference.

So the moral of the story is, if we would all just drive a little more even-keeled, we could all save a LOT of gas.