Probiotics are important to repopulate you intestinal flora. I'd start a 2 week course when you start your soup. I use Udo's Super 8. I get them at my local health food store or Whole Foods. They are in the refridgerated section. They cost about $15 for thirty capsules. I take one capsule at night. There are other probiotic suppliments that are equally effective, I'm sure. You can also eat/drink kefir each day. Kefir has several strains of probiotics in it and it is tasty in a smoothie. I usually have a smoothie each morning with it.
Also be sure to continue to drink a lot of water. The recommendation to drink half your weight in water is not eclusive to the Master Cleanse. As a matter of fact, that is the recommended amount of water for healthy (cleansing or not) people to drink on a daily basis. Most people are actually dehydrated. Hopefully, while you were on your MC, you learned to be fully hydrated and will continue in this healthy habit.