dust filters of course, but having positive pressure (ie more air going in than going out) means that you can stop dust from coming in small vents and cracks because all airflow is being forced out of them rather than being drawn in. All you need to do then is filter the intakes. I still have a layer of fine dust on the floor of my case, but nothing like the big clods that built up on my cpu cooler before I configured it this way.