Fill it with salt and rubbing alcohol. One of my friends has a glass bong about a foot tall, I take out the slider and clean that seperately (it can be boiled), plug the hole where the bowl goes and pour in a good amount of salt followed by a fourth to a half bottle of 70% ISO alcohol (depending on the side of your bong) make sure all holes are covered and shake up and down for a few minutes. You can repeat this process until the bong looks new again.