There is an ancient technique practiced in India called Shankhaprakshalana. It is basically a salt water flush with some yoga postures practiced between drinking the salt water solution. There are differences of opinion on how much water to use and how much salt to use, but I learned from a native in India who really seems like they know what they are doing and they used a very large amount of water maybe 4-5 liters. The trick for getting the correct amount of salt is to make it equivalent to the amount of sodium that is in your body. The way to do this is to taste the amount of sodium in your body by putting the pinky finger in your nose and touching liquid mucus, then taste the mucus. It sounds gross but this mucus is indicative of the amount of sodium in your body. Then continue to add salt to the water until the taste is the same as the mucus. This will be quite a large amount of sodium.
If not enough salt is added to the water, the body will ingest it through the intestinal tract instead of letting it pass through. If the amount of salt in the water is equivalent to the the level of sodium in the body, then the body will allow it to pass through while only assimilating a minimal amount. Of course you don't want to use too much sodium either. As with all cleanses you have to be careful and not do it too often as this can deplete electrolytes.