Unlike pretty much everyone else so far, I'm not going to mention diet! I think most of us that get obese have tried many diets so dieting isn't the whole answer.
I reckon many of us know what we should be doing diet wise, so the question is why is eating too much/the wrong things more important to us than being healthy?
If we just think about diet, we'll never know why or challenge that negative mindset to allow ourselves to maintain our control, and therefore size, even if we have successfully 'dieted'. That's why so many dieters regain.
So my advice is to keep a thought and food diary. In it you record EVERYTHING to do with eating. By that mean the thoughts about food, even if you don't actually eat.
You need to be recording when you think about food and the context of that. E.g. Where you are, who your with, what your doing, how you were feeling, what foods, how much consumed etc. I had mine set up in columns to make sure I had all this covered.
It wasn't too long until I spotted a pattern of my triggers. I was challenged at 2 periods in the day. The first I was alone, bored, feeling down on myself. The second was in the evening after dinner, with my hubby, relaxed in front of the telly and he'd eat too.
It helped me to know I actually was ok the rest of the time and just needed to focus on making changes to cope with these times.
Those who successfully diet and maintain, their issues will have been knowledge based such as portion control or the types of food and a diet with help with that.
For the rest of us who know what a healthy diet is but can't stick to it, understanding our triggers and combatting them is more important before starting a diet.