The most effective diet I was ever on was the 'not exercising' diet.
For several months due to an injury I couldn't excersise. All of a sudden I couldn't justify a slice of pie or a large soda pop with the mental excuse "eh, I'll work that off at the gym/court/trail"
In fact, it was the first time in my life I started to really think about and notice what I ate. It was paradigm shifting.