As usual, I don't agree with anyone; at least I don't think I agree with anyone. Since you say you're quite thin, I'm assuming that the reason your stomach isn't flat is due to poor posture. Dieting or doing exercise to lose weight from a specific part of the body is a common myth that many people believe works. Doing crunches or sit-ups or any other exercises for your stomach won't make if flat unless you are in terrible shape and all of your muscles are weak. Since I'm assuming you are young and fairly active I suspect that's not the problem. Correcting your posture, walking upright (as opposed to slouching) and consciously contracting your abdominals will almost certainly work. The problem is, of course, that it takes a lot of effort and concentration until it's a habit and you do it without thinking about it. I worked out a lot when I was a teenager and I had the same problem even though my abdominals were very strong. It was only when I consciously improved my posture and made the effort (stomach in, chest out) that my stomach became very flat and people remarked about my six pack. Of course all this is conjecture because I haven't seen you, but if I'm right about your posture, I'm probably right about your stomach too.