My mom yoyo dieted for years trying one fad after another and they all worked but once she reached her goal she stopped eating that way and ultimately put the weight back on. And sometimes even more. Not this time though, because this time she didn't try any fads and she didn't really give anything up to the point where she got to go "off the diet" once she reached her goals.
In less than a year she went from 181 to 135 and from a size 12 to size 8. All by watching what her portions, no exercise whatsoever. I am still trying to get her to at least take walks in the evening and I am hoping that she will eventually start doing that.