Personally I believe it is madness to fast without doing some sort of colon cleansing routine every fasting day. Virtually every fasting clinic will agree, as do all of the pros I have read or talked to.
Which routine you use is largely a matter of choice. Burroughs of the Master-Cleanse doesn't like enemas, while the legendary Paavo Airola swears by them. I have also read the water forum which seems to think neither is "natural" --
The concept, and the reality from my experience, is that every fasting day your body is throwing off toxins like mad. The main escape route is the colon. On many occasions I have had a plain water enema stop a detox crisis dead in its tracks, and it is still my daily routine of choice particularly on longer fasts, as it gives me control over when and where I do my thing.
The Master-Cleanse uses a combination of herbal lax tea and/or the fabled Salt Water Flush (SWF). It only took me four years to figure out how to do this easier method:
Dissolve 2 tsp. uniodized Sea Salt in ONE cup drinkably hot water
Have THREE cups plain water standing by
CHUG as much salt water as you can, chase with fresh til ALL water is consumed
About 1-2 hours later the entire quart will exit, carrying everything within its path. This is effectively an "oral colonic", a good flush of the entire digestive system. The Master-Cleanse recommends it daily for the ten day fast; I prefer to use it only occasionally during a fast of any length as I find it a bit harsh for daily use and inconvenient unless I plan to be home all morning.
The tea... drink a strong cup evening and morning to flush the colon while fasting. Personally I don't care for the tea, but lots of folks like it.
Another opinion: I have done all sorts of fasts, the MC, fresh juices and even water. The MC is wonderfully convenient and very effective, a good choice for first timers. Juices are more supportive but also more work. Water should be reserved for those who are already pretty darn clean, and who have prior fasting experience. Be aware that water leaves you pretty weak and worthless, while I have done my usual gym workout every fasting day, even on my 60 day mega fast, barring crises. During ANY fast, should you hit a crisis, treat yourself as though you have the flu and lie low.
And NEVER break any fast on a low. Fast through until you feel well before breaking.