Dust is always a problem with electronics, and well, life in general. But most importantly, its a problem with computers. I've tried various means to stop dust from entering my home, including a HEPA filter and always keeping my air conditioner circulating the air, but nothing works quite well enough. Eventually I came to the conclusion that you have to tackle dust at the computer-level, and for that people resort to a variety of air filters.
Traditionally, these filters fit over your fans (either snapping on, screwing on or being magnetic), and block dust intake. The problem usually lies in that these filters tend to restrict airflow too much, or not be effective enough, usually either one or the other as they are closely related. When I built my newest computer some months ago, it came with those metal screen mesh-type filters and I thought "Hey, these will work pretty good!". Well about a year later I opened up the box to add some new parts and everything is clogged with DIRT.
If you have a computer like mine, which weighs so much you can barely lift it, fits a CPU cooler the size of a car radiator, and has a graphics card that is as long as a bus and cooks omlets, along with a billion cables going to half a dozen different drives and a myriad of fans to keep it all cool, dust is your worst enemy, because the dirtiest spots are so crowded that you can't even get to them with a toothbrush.
I've completely given up on any kind of aftermarket filter, or even the ones that come with computer cases by default. What I have found to work the best is dryer fabric softener sheets. This may seem strange, but I tested the material for the purposes of air filtration on some junk PC that I was building about a year back, and I noticed that afterwards, the inside was always completely spotless.
I repeated the test on subsequent computers that I built and all of them had the same behavior - no dust inside, even after months of operation. And I also noticed that despite this, the airflow from the fans wasn't all that restricted when properly installed. This is because despite being fine enough to allow air to pass through them, dryer sheets also seem to have some kind of "static cling" quality that causes smaller dust particles to stick to them, so even particles that are too small to be blocked by the filter still don't make it into the computer.
Note that any existing (i.e ineffective) air filters, covers and screens on the system should be removed to not further impede airflow.