A can of compressed air and a small brush is all you need. Just give the insides a dusting with the compressed air at whatever interval you want (6 months, 1 year, etc) or whenever it gets too dusty for your taste and use the brush to get into the small spaces. I usually clean my computer once a year, but I have dust filters so my insides don't need to be cleaned any more often.
If you have access to an air compressor, you can use that with the blow gun attachment. Just be careful since the air comes out at much high velocities than from a can, so make sure to stay a greater distance away. I just cleaned my computer out with the air compressor and it does wonders.
And when you dust the fans, make extra careful you don't let the...