It is a good idea to start when you can take it easy. During my first Master-Cleanse
I was so tired and nauseous that I had to take off day 3 as a sick day. I also fell asleep at my desk at work during the afternoons of days 1 and 2. But then day 4 was great, and so was the rest of the MC.
I don't think there is a lot to prepare for other than mentally. My feeling is that attitude is the most important part of the MC. I don't believe anyone can die from drinking lemon juice, maple syrup, and cayenne pepper for ten days, so the main thing is getting through the worst of the detox symptoms (for most people, days 1 - 3) and then dealing with the emotional aspects of not eating for the rest of the MC.
The thing that helped me the most in my MC's was to take it one day at a time. Plus, I kept telling myself I had the rest of my life to eat - I was only doing this for an extremely short period in the grand scheme of things. Finally, once you get to day 4, you will probably find it easier NOT to eat then to eat - at that point, your body has adjusted to not eating, and if you DO eat at that point, anything other than juice or broth, you will most likely get sick. So it's easier just to not eat for the remainder of the MC.