I'd have to go with gasoline.
Look at how many calories that stuff contains. Almost 1100 per 100g. Cut that out of your diet and you're set.
Seriously mk II:
This article is a good approach to one side of the problem, but as many of the commenters have noted, exercise is key. This article is about building effective habits, so how about a companion article on effective habits for incorporating little bits of exercise into your life? I don't mean "go to the gym"--I mean things that you have to do anyway that can either help or not.
My favourite: gasoline makes you fat. Gasoline-powered personal transportation is for geriatrics. That stuff will kill you. However hard you try to commute by bike, do errands on foot, etc., try a little harder. Never skimp on cold-weather biking clothes, panniers, etc. Make your bike genuinely useful. Then leave your car at the mechanic's for a week and see just how much you don't need it.
Other things I'm just making up as I go along:
- Don't use a mop to clean the floor--use a sponge. Handles make it too easy.
- Forego power tools. A hand-push lawnmower, coffee grinder, rake, etc., will all give you a little extra exercise.
- When tying your shoes, or washing your feet in the shower, don't lean against anything. Pick your leg up.
- Remove your staircase. Replace with a rope ladder, or maybe a wall covered in climbing holds.