Fresh juice is always best. That being said, preparing it in advance is MUCH better than not doing the MC! I have made up to a 1/2 gallon at a time and take it with me. Others have said that premixing the cayenne makes it even spicier, so I guess it depends on your taste. Some make the juice with out the cayenne and add it to each glass. Others just mix the lemon juice and maple syrup, and add the water/cayenne for each drink. I put my cayenne in capsules and mix the rest in advance when I'm to be gone for a while; either 2 or 6 drinks at a time.
There's a recipe for making it in larger quantities at the link below. Feel free to cut the amount of maple syrup if you're trying to lose weight. I use 1/2 to 3/4 of the amount called for, but not if I'm feeling really hungry.