Well, bongs get grimy - if you are no big fan of patina you should clean your bong after a few uses. Special cleaning brushes for the bong and oriental waterpipe are a real winner when it comes to this.
Brushes for bongs...are highly recommendable for a proper clean pipe. Take care you choose an adequate length to make sure it will reach the bottom of the pipe. In addition, the shape of your bong is essential. There are special brushes with expandable bristles for bongs with bowls. Otherwise you'll probably fail to reach the filthy parts at the inner side of the bong, close to where the bowl goes.
Downpipe brushesIf you dont want to use your standard cleaning brush or pipe cleaners for your downpipe, there are special brushes for downpipes chillums too. Same things goes as with the big brushes: check your length.