Some people can do this fast to lose weight, but I personally think it is too hard. The only way I stayed on it was because I truly believed it would help me become healthier (and fortunately it did!) If you think about it that way - that you are making a short term investment by doing the Master-Cleanse for your long term health, it might help. Also, you can't do it alone (or at least I don't think most people can). You need help from whoever or whatever you believe is greater than yourself, whether that's God or a lemon tree or this forum. :) Think about it like you are going on an exploration of your mind and body and you will find it very interesting how your mind and body changes throughout the MC.
Just some thoughts. I'm trying to gear myself up for a 21-day Master-Cleanse starting June 3, 2004, so I'm a little more philosophical than usual. Good luck!