Step one in any diet or budget plan is "pay attention". Write down everything you eat or everything you spend. Whatever you're trying to control, you need to track it. But tracking is not enough. You can't just track what you're eating and hope for the best. That collected data is just a step. To make progress, you need a plan. You need to set limits on what you spend, and bring the pain of spending off your budget closer to the enjoyment of the moment. When you spend from a credit card, the pain comes at the end of the month, and until then, you have the pleasure of the purchase. To change your habits, it helps to make that pain immediate at the point of purchase. You need to see your balance whenever you contemplate a purchase.
This is where mobile apps can help. Imagine that all of your purchases went through an app that also monitors your budget. You have $100 you are allowed to spend on groceries this week, and when you get to the checkout, that number comes up and your purchases are deducted from it. You see imediately the bite you have taken from your budget. EFTPOS doesn't show your balance on the machine, for good reason, so this would be a step up.
But maybe that's not immediate enough. You might fill your trolley and only realise you've made a mistake at the checkout. What you need, then, is a budget app that helps you track a trolley of shopping as you stack it up. Then every time you pick something off the shelf, it comes off the budget. You'll see the damage you're doing that very second. That's real immediate pain at the point of the decision.
For dieting, this is not so easy. A daily weigh-in isn't enough. You need to see your projected weight gain with every bite you put in your mouth. Perhaps something like Google Glass connected to pressure sensors in your shoes. Every time you stand still you'd see your current weight. If you eat standing up, you'd see the number going up the whole time. That's bound to make you think.
Mokalus of Borg
PS - Though, as demonstrated by the faults with BMI, weight is not quite the right thing to measure.
PPS - I'm sure we could arrange a belt to measure your waist at all times.