Many people spend their lives always telling themselves they are about to go on a diet. I believe these people are bound to get fatter.
My theory is that the body is pretty smart and if it knows that it is about to suffer a period of low calorie intake it stores away energy in the form of fat. If however the body knows that it has a steady supply of ice cream and donuts heading its way it will just convert all that unneeded stuff in to poo.
If you want a diet to be effective make it a snap decision - or just don't diet at all. You'll probably be happier and thinner.