If you don't have your car checked out every once in a while, your car runs less efficiently and you spend more on gas. Make sure your tires are inflated to the proper pressure. If you live near a bus or train stop that can take you to work, consider using it. You will probably spend more in gas and car maintenance than the monthly pass will cost you. Look into carpooling.
Most people drive to work alone.
Driving with one or two others takes that many more cars off the road. If you have more than one errand to run, combine your trips and think about the route you will take to minimize your driving. If you have anything in your trunk that you don't need, take it out. The added weight reduces your fuel efficiency.