I'm always opening up my case and changing things, some times I don't even bother putting most of the case back together again, so keeping it clean is pretty easy.
Can of compressed air works really well to get rid of loose dust, for those slightly older components that are really bad you can get some anti-static wipes that works like a charm.
However, unless you have a good Power Supply Unit that cost £35+, you'll probably find that because it only has 1 small fan hole, a hell of a lot of dust will collect in there and distribute itself among the case always keeping it dusty. It is possible to rather carefully unscrew it without bending any of the metal casing, then a hover, a can and some wipes are generally needed to get rid of everything. Power Supply Units are always a pain, manufacturers put low quality ones in cases that have a high chance of dying out compared to other computer components, however they can cause the most amount of damage to your computer when they go wrong.