Believe it or not, the screen on the back of the vent will stay pretty clean just by the normal and natural blowing winds. But if dust starts collecting from low speed winds or times of no winds blowing, a shop vac can be used from inside the attic and gently touch the screen to suck the dust off. If too much builds up in the tiny perferations it can block the benneficial airflow that is needed to cool the attic. Once a year it is a good idea to clean your dryer vent and ventahood filters. This would also be a good time to look at the screens on the rear of the gable vents. But never use a damp rag to remove this dust as it will actually create an enviroment to attract dust and make it cling more.
Another common blockage issue is storing boxes in the attic. Anything that stops or detours airflow in an attic or loft can affect the ventillation. Having large enough gable vents will still do no good if they are blocked. Also, cardboard boxes and Christmas decorations make good kindling for high temperature attics to spark and catch fire. Temperatures reaching 160 degrees can easily light it up. And after an attic goes up in flames, the entire home is destroyed. Spiderwebs or cobwebs can also be a hazzard in attics. Does this mean clean your attic everyday like your home? No. Checking once a year or having a vent cleaner (HVAC servicer) come in once a year to check the basics of the home will do and usually is called a prevention inspection. Just for the safety of your home and family just like taking your truck into a shop for an oil change and they check your transmission while there.