I despise those liquid diets. I'm hardly an expert on this topic- I can't give you facts and figures, but I can tell you this much. I've lost about 48 kilos in two and a half years. I have another 15 to go, before I'll be happy. I started my weight loss thing with a friend who went the shake route. She lost a lot of weight very fast, but found it really hard to maintain. Without maintenance yo-yo weight swings are in fact worse for your health than being overweight.
In my experience, plateau's are completely normal. Each body is different of course, but for me there are two types of plateau.
1. My body needs a break. My doctor explained this to me in medical terms- but here goes my layman explanation. My body was used to being X weight. I then lost 15 kilos and now my body is stressed trying to put the weight back on. My doc said that it usually takes about a year of maintaining weight before the bodies' natural urge to maintain at the old weight fades, and the new weight kicks in as the 'natural weight'.
For me, this was characterized by utter exhaustion. I felt like I was battling myself for the energy to breathe. Usually, after exercise I would feel a small rush of adrenaline- during this period I just felt drained and emotional after exercise.
In the two cases where that happened, I stopped aggressively pushing for weight loss, ate food at a level where combined with exercise I would simply maintain my current weight. After a few months of this, I would gradually increase my exercise/decrease my food, and found fat began to melt off once more. More importantly, I had energy once more, and felt healthy and looked it too.
2. Your body has grown used to the exercise you're doing. Especially if your exercise is incidental -i.e. "I need to walk up that hill to buy some milk!"- this is likely to be true. You need to find a way to switch up the level and challenge imposed by your exercise.
As most of my exercise was done at a gym- I did one of a few things when this happened:
a. I tried changing machines/levels/amount of time. For example, my machine of choice was always a cross trainer. I went from using that predominantly to using a row machine and found weight began to come off again.
b. I went to classes- while I could slack off on the rower, I felt I had to keep up when I was in a room full of people exercising hard. Spinning classes are particularly good for this in my experience.
c. I changed the amount of weights training I was doing. (as an aside, please do weights training- even if you don't go to a gym, lift a couple of cans of tomatoes or something. Without that, you can lose bone density during weight loss, which can leads to problems like Osteoporosis down the track.)
d. I picked up another activity like Dragon Boat racing- with practice, it was about 3 hours of exercise a week, fun and kick started weight loss again. I signed up for 3 months.
This is a loooong response. Sorry if it's preachy. I just really feel strongly about those weight loss supplement things. I would suggest you look at why your weight loss is halting, and then look for solutions- and go for the healthiest option, even if it does mean the weight loss doesn't happen as quickly as you'd like.