I know many people aren't fans of a vacuum cleaner but I have always used that to clean out my case. One wants to be careful and not bring the nozzle head into direct contact with anything and I always reduce the suction as much as possible. With things like fans I gently hold my finger on fan blades so they don't spin while I'm drawing up the dust. This has always worked to get rid of the vast majority of dust and it has never cost me a penny. It isn't perfect but perfection isn't required. Just be careful and don't go in like a wild man.
