Juice fasts are good for resting and sort of "resetting" the digestive track.
It depends a lot on your personal situation and what your body likes and doesn't like. For me, I don't tolerate most grains at all. So while some people might get some awesome bowel movements going from eating a lot of oatmeal, for ME, oatmeal would sit in my gut like cement and leave me constipated and sick for weeks.
So that's why I think before attempting any sort of cleanse, one would need to first figure out the optimal diet for their body. Grains, no grains, meat, no meat, any food sensitivies, etc.
When I first got into alternative health I attempted a lot of colon cleanses and stuff which involved stuffing my insides with tons of fiber but I've since learned that isn't always the best thing. Some fiber is important but for me tons of fiber does more harm than good. It really clogs me up. I learned a lot over time about my body, and that is how I got to the point where I really saw how fasting on juices healed my insides much better than stuffing them with fiber.
If you've never cleansed before, I recommend starting with diet. Add more salads, greens, basic fruits, etc. See how you feel. Keep it simple. Maybe do an elimination diet to test for any sensitivities. A naturopath friend told me that one should be on a simple diet with lots of healthy raw foods like salads (loaded with artificial preservative filled dressings = doesnt count, make them all organic and natural) for awhile before attempting any sort of cleanse. She also said that drinking a few cups of red clover tea a day can help get the body into healing mode before attempting any major cleanse. I really think she is right because cleansing can unleash such a scary amount of toxins all at once that it's better to start slow.
Overall, Id say
1) remove all processed foods
2) eat raw and organic
3) eat enough protein for your metabolic type, whatever that may be,
4) figure out any individual sensitivies and remove those foods if necessary (dairy, glutens, etc)
5) make sure to get healthy bacteria into your gut (kefir, kimchi, etc)
6) Then you can start doing cleanses such as liver, gut, parasite cleansing, etc.
Parasite cleansing is REALLY important but you want to get everything else in order first before tackling all that, if it's your wish to do so. Cleanses are crucial but diet has to be right otherwise it's like mopping a floor and then dumping paint all over it right after.
Hope that helps!
