Hm, difficult question. They say people who become overweight have a genetic disposition for it, while people who keep their weight are nervous types who burn part of their calories through lots of small movements - shifting in their seat, talking with their hands, scratching themselves, etc. (they actually tested this hypothesis by observing subjects with a hidden camera). So those are two things that none of us can change. I had the opposite problem, I was too thin for my size, and found that the two foodstuffs that made me gain most weight were a) alcohol and b) fat. Sweets that contain little or no fat (like cotton candy) don't affect my weight.
I suggest drinking nothing but water and eating lots of lean meat (possibly marinated so that you don't need sauce or garlic butter to flavour it), roast potatoes or rice, vegetables and wholemeal bread with low-fat butter.