There is no simple answer. Colon cleansing depends very much on individual situations. For example, it took me four years of concentrated effort and lots of long juice fasts and many many professional colonics before I achieved my colon cleansing goals!
If you are pretty clean, there is no need to do any pre-cleansing. If you want to do this it definitely makes the first few days of a juice fast easier -- simply eat nothing but fruits and vegetables, and lots of water (fresh juices and herb teas if you like) for a few days or more prior to beginning your fast. If you drink caffeine, taper off over about three days before stopping to avoid the dreaded caffeine headache.
The Master-Cleanse itself is a wonderful colon cleanse. Giving the digestive system a ten day break allows it to achieve many wonders.
Yep, it's work! And yes, it is worth it!
Whatever your goals, if I can do it, YOU sure can too!