Use public transportation or carpool at least once a week to get to work; save on gas and save on parking.
Obviously the amount you save by carpooling or taking public transportation (or walking, if you are able) to work one day a week will vary depending on your circumstances. Calculate the amount of money you save on gas, parking, wear-and-tear on your car, etc. and put it in a jar each day you don’t drive as you regularly do. Watch it add up!
ADDED BENEFIT: Remember that carpooling can make for good company and conversation, which can be a nice way to start and end the day, and don’t forget that any extra walking you might do in order to take public transportation provides a health benefit as well!