Drive less. Use public transportation, carpool, walk, bike, or however else you can avoid using your car.
When you fill up your car, go to a station that has vapor recovery nozzles--they help cut down on air pollution in a big way.
Keep your tires properly inflated, and don't top off the tank.
See What you can do to reduce air toxics, from VT ANR

Links

Resources and Tools

How much does your vehicle pollute? by Environmental Defense
Calculate fuel consumption, vehicle pollution generated on any given trip, and pollution generated by lawnmowers and snowblowers. From Environment Canada's Environmental Technology Center.