Changing out air filters every couple months is the most important task for keeping your air conditioner, furnace, and vents clean and operating efficiently. However, dust will still find its way through or around filters over time into your house's duct work. Once a year it is a good idea to do a more thorough cleaning of the air vents.
Open the access panel and remove the air filter. Use a vacuum to clean dust off all the reachable surfaces. Make sure to get all the build-up around the edges of the air filter slot. Next, clean the vent cover (a.k.a. grill) with the vacuum. Use a mixture of water and a little bleach to remove any build-up left after vacuuming.
Make sure you clean both the air intakes and returns. Only the intakes should have air filters and replace them at this point if they look too dirty. Ensure the filter is inserted the correct direction when replacing. There should be arrows on the side to indicate the direction of air flow.
Keeping your home's central air system clean will help it to run more efficiently. The more dust that builds up on the evaporator of the air conditioning systems, the more energy is required to cool the air. Vents plugging up from dust also slows the air flow and allows energy to dissapate.