Keep it off the floor, positive air pressure and compressed air every now and again is about the best you can do.
I knew someone once who set up a fancy air intake system using hoover tubes that drew air in from an air filtration machine, so no dust could get drawn from inside his room. From the little I remember, it worked well but cost a lot, and made his room cold! :P If you're innovative and have money to burn you could experiment with something like this, but you need to find some way to make sure there's enough air coming in to cool the system (because there'd be a reduced intake, you'd need to compensate by keeping the air moving quickly.)
Anyway, whatever you do, remember that it's something that affects mostly all gamers, so you're not on your own. I for one live in an old cob house, with my computer on the floor in my room, so dust for me is a real problem