Personally, I wouldn't diet! A diet may get you slim quickly, but I don't believe that it will keep you slim for long, a healthy lifestyle is key for me.
If you want to be slim, I'd say that you need to live like you're slim, and living a slim persons life (as far as I'm aware) doesn't involve pigging-out all the time, it involves living a healthy, active lifestyle.
I think the short bottom line is: Eat and exercise with the vision of being a slim person, I think this will also help you make the right decisions when it comes to eating etc.
Nick