More Saving the Planet...
...while being really lazy. Cleaned out your car recently? A lot of people use their cars as trash cans. So drag a trash can out to you car and empty out the junk. Your car will smell better (with all the benefits this entails) and you could get quite a bit more mileage too. Here's a quick story: this morning with one passenger and his luggage in my car I got 38 mpg on the highway. Then I dropped him and the luggage off and with just me and no luggage I got 42 mpg. The conservative estimate is that every 100 lbs costs about 2% of your mpg but is actually even more significant for smaller cars like my 4-cylinder Malibu. Bottom line : if you've been stowing 100 lb of junk in your car you'll see a difference (get out a scale and start pulling out your ancient CD's, tapes, 8-tracks, the map of the grand canyon you'll never use again, and starting looking under those seats...). Then go to www.fueleconomy.gov for more good tips. Combine driving style with low weight and you too can get near-hybrid like performance out your regular car like my 42 mpg malibu.
I feel like there hasn't been a lot of science here, so one BIG deal in fuel economy is the drag due to wind resistance which is why you can see big improvements in your mileage by not speeding. Click and Clack (who have a lot of good advice on fuel economy) mentioned in one of their shows that wind resistance goes as the square of speed. Doubling your speed quadruples your wind resistance. Aerodynamics is so crucial to fuel economy that Mercedes designed this concept car based on the oddly well shapped boxfish and the resemblance is spectacular (as well as the coefficient of drag!). I love that you can look at the car and get a total flashback to which fish it was based on.
I'm never all that comfortable about citing wikipedia although it looked correct. It was also a little dry and boring. Here's a great racing article that explains terrifically (and with a good citation) not just that wind resistance is a square law, but that horsepower is a cube law in the speed of your car. If it takes about 7 hp to get to 55 mph, you need 31 hp just to get to 90 mph.