I always do this, and it's generally an unconscious process. I pick a habit to take up, or a bad habit to stop. I set a deadline. It all seems reasonable. And then I do the exact opposite, in the most extreme manner possible. For instance, if I think that I will go to the gym everyday for a week, I can be certain that I will not go to the gym at all, and that I will move as little as possible. Or if I decide I won't eat sweets, the next day will find me consuming every sugary confection in sight. Yesterday, I thought to myself that I would try being vegetarian for the rest of the month. I know how to be vegetarian; I was meat-free for almost 8 years.
So what did I buy at the grocery store today? Beef pinwheels and smoked turkey lunch meat.