1. CHANGE THE AIR FILTER! This alone will reduce the wear on your blower motor and make it last longer. It will also keep the cooling coil (evaporator coil) cleaner and more efficient. When do you change the filter... usually every time you get a light bill!
2. USE A PLEATED FILTER. The fiberglass filters do little good at keeping dust out of your system, ducts and ultimately your home.
3. Have a Licensed Contractor check your system AT LEAST annually. If you live in a harsh climate, a semi-annual check would be advisable. In Texas, I recommend a Spring and Fall checkup.