Ensure that your car receives regular maintenance and oil changes. A good guideline is to follow the maintenance schedule supplied by the manufacturer of your vehicle.
Make sure to have your emission controls systems checked on an annual basis. Be careful not to tinker with any of your vehicle's emission control devices.
Other Suggestions for cleaner air include:
Use a bicycle or walk as often as possible (leave your vehicle at home)
Try to use public transit several times a week
Join or set up a car pool for work or activities (i.e. your child's baseball team)
Plan your errands so that you need to do as little driving as possible
Be careful not to overfill your gas tank
Ensure your tires are properly inflated
Make an informed decision when choosing your vehicle. Although mini-vans and light trucks can use a lot of gas, they can carry several passengers or a fair amount of cargo. Consider how often you will be the only person in the vehicle.