As evilgenius134 mentioned,
Positive pressure is as to what I tested one the easiest and best ways to keep dust out. The air pressure acts as the sealer of the case which prevents more crap/dust from entering. I did it and its a huge difference. Since I have a fan controller, I simply turned my main intake fan to higher speeds and my exhaust fans to lower speeds.
Note: Cooling will suffer a little bit due to this, but I believe positive air pressure is still better cooling than installing intake filters which choke airflow not just normally but even more so when they get dusty.
Once it gets a lot of dust, best way to clean is use a microfiber towel to wipe the basic surfaces and use compressed air to clean the components. I highly advise against using your own air compressor, use the cans they sell in stores. A regular compressor has air that can have moisture and be dirty. Air compressed cans are clean.