if its a really thick caked on layer of dust then use a vacuum, because you dont want any respiratory problems.
but i usually just use a can of compressed air or my air compressor.
Compressed air is flammable to a point (if you hold a match or lighter directly to the air stream). Definetly isn't a problem for your case. Use it and make sure to blow dust out of all the fans and small compartments in the case as well.