Compressed air canisters are probably about the best you'll get for cleaning. Don't use a vacuum as they can case electro-static damage.
However, it's fairly easy to prevent a lot of the dust getting in in the first place.
Try to keep a positive air pressure - ie, more fans intaking than exhausting. This means air is blown out of any vents in the case, and if air is blowing out, dust isn't going to get in.
If you do that, you need to make sure that all the intakes are well dust filtered, otherwise you'll just end up with those sucking dust in.