1. Use the cheap filters, not the expensive three-month fancy allergen filters. The air moves more freely through the cheap ones. Change it monthly IF you have animals in the house.
2. Hose off the outside condenser. It gets dirty out there with dirt, grass, pollen, etc. and if the little vents get too clogged with dirt you'll get a slow-down in the cooling and eventually things will freeze up and you'll have no cool air in the house. Just get a garden hose and squirt down all the sides of the condenser - you'll see the dirt pouring out, especially if the condenser is outside the house, on the lawn, where the grass clipping, etc. go flying by all the time.
