I don't know how well any of these other methods work but I can tell you that Oxi Clean works amazingly well. Just plug the downstem with wadded up paper towel or something, fill the bong up to the brim with hot water and drop in a scoop of oxi clean. Wait a little while. If you have a clear bong you can actually see the oxi clean fighting the resin. After this is over, rinse the bong out several times with warm water.

PS - Be careful of drastic temperature changes of your bong, as this can cause it to crack.