amimagic - I was a size 14-16. Then I started a program at a sports hospital here, where I was followed by a doctor, a dietician, and a trainer (they had an in-house gym). In three months, I lost 10 kgs and went down to size 10. That was 3 years ago, and I haven't put on any weight.
I went into 1st meeting with the dietician saying "I can stop eating carbs, no problem" and she said, "No, actually, you will eat a bit of bread, pasta, potatoes, rice etc with every meal". So I did. And I lost loads of weight. And I kept that weight off.
You should eat without depriving yourself, but being careful with the amounts you eat. She showed me that I don't actually have to eat a plate of pasta, and I don't have to have pesto sauce on it when I'm perfectly fine with a simple tomato sauce. That half of my plate should be salad or vegetables, because eating is also about volume - filling your stomach - but that I should put only a tiny bit of olive oil on the salad. That a quarter of my plate should be protein - a steak, one egg, a chicken breast. That the remaining quarter should be carbohydrates (slow sugars) like rice, pasta, 1 slice of bread and that will keep me from raiding the cookie jar later in the day.
You also have to exercise. It is not even just the calorie in/calorie out calculation. Cardio exercises increase your metabolism. I run for about half an hour, twice a week. It is not much but I can feel the effects - for the rest of the day, I breathe differently and I stand differently. And my tummy goes in.
Anyway, that is my two euro-centimes.