Everyone knows dust is bad for your PC, but it's hard to avoid. Apart from regular cleanings, the best way to avoid dust buildup is with a good fan filter. If you don't feel like spending money on them, though, just use some old pantyhose.
The above video, from Linus at Techquickie, discusses the optimal setup to keep your PC dust-free. We've talked about this before, but for those that have forgotten: ideally, you want slightly more intake fans than exhaust fans, with filters on all the intakes. That prevents dust from seeping in the other cracks and holes, while ensuring no dust comes in through the intake fans.
But what really caught my eye is the tip at 3:35: if you don't want to spend money on filters (which, if you have more than one or two fans, gets pretty expensive) you can just attach some old pantyhose on the outside of your fans. I checked around, and quite a few people have used this tip with success. So grab some old pantyhose (or steal them from your girlfriend) and enjoy your new dust-free machine.
Positive vs Negative Air Pressure as Fast as Possible [Techquickie (YouTube)]