An easy way to get better gas mileage is to make sure that your tires are properly inflated. Almost nobody checks this and a recent study found that, on average, car tires are usually at 80% of their suggested pressure (http://www.cmu.edu/cmnews/extra///050921_tire.html).

A general rule is that you lose 0.5% of your mileage for every PSI that your tires are under-inflated. This may not sound like a big deal, but it adds up. An average driver gets 22 miles per gallon and drives 12,000 miles per year, and the average tire pressure is 32 PSI. So the average driver is using about 20 extra gallons of gas a year due to low tire pressure. At $2.50 per gallon, you're not just helping to save the planet, you're saving a nice chunk of change too.