I lost a lot of weight a few years ago and kept it off for ages...what did for me was having a baby plus being very ill afterwards and existing on cake and sugar for months at a time. Thankfully the craving for cake stopped after a while! 
But I'm now a few stone overweight still and find it difficult to lose, because though I eat very healthily I just eat too much of it (don't snack, cook from scratch, eat tons of veg, very healthy stuff - a typical dinner would be tomato, carrot and chickpea casserole with brown rice and sliced runner beans - but then I eat two and a half platefuls of it....whoops.) My DM does SW so I looked into it, but it hardly makes a difference from my usual diet (I couldn't believe that DM was going on about how I could eat as much brown rice, pasta and plain potatoes as I like - no, that's the problem!! Portion control....)
When I lost weight before I found the best combination for me was not getting too hung up about it, but combining low carb with low calorie - so I'd have one meal a day which was calorie counted, and one meal a day which was low-carb but not calorie restricted. Plus as much coffee as I liked, fruit for breakfast and no more than one glass of wine in the evening. If I had a calorie counted lunch (say a sandwich of 400 calories), I'd have a low carb dinner with a salmon fillet or chicken, plus tons of veg and a glass of wine. If I was out for lunch I might have it low carb (meat and loads of salad), then have a calorie counted ready meal in the evening. Unfortunately, I'm small, and my resting metabolism requires pretty low calories (around 1500), so I found I had to go down to approximately 1200-1300 to lose weight at a steady 2lbs a week. That's not a lot, but to be honest I hardly noticed once I'd got into the swing of it - because I wasn't being too obsessive about it and the low-carb meal allowed me not to calorie count but to eat as much veg and salad as I wanted.
Once I'd lost the excess weight, I could go up a bit to a slightly larger calorie intake; but to be honest I could never fall back into eating very big portions or loads of carbs again without putting weight back on. I think that's the problem with SW and other diets generally: there's no getting away from the fact that you have to switch to smaller portions permanently. We live in an age where there's an excess of food available, and even if you eat very healthily, there's no getting away from the fact that we have still got accustomed to far larger portions than is actually needed by most modern lifestyles - or even most high-activity lifestyles! (I mean I could work on an oil rig and I still probably wouldn't need two and a half large plates of brown rice, chickpeas and runner beans....)
Now, this summer I'm hoping to crack on with the proper weight loss plan again...wish me luck.....I might have to buy smaller plates.... 