I did a 3-day detox, which involved juice fasting. I lost 11 pounds but gained back about 6 of it when I started eating again. There are different detox methods - some involve fasting and some don't.
Here is a good link that describes a 7-day juice-fast & detox.
Juice Fasting
My personal experience with the 3-day fast & detox was pretty positive - You get a lot of crap out of your system - Figuratively and Literally - LOL. But you really do feel great afterwards. You just need to make sure that you're able to do it (ie no medical issues,...etc). For me I only got REALLY hungry about 3 times in 3 days - so it wasn't too bad