Stay away from Slim-fast and other so-called "healthy" candy bars and shakes, look at the nutrition information and find out why. A Slim-fast shake contains around 400 cals or more, you'd be better off eating a chocolate bar. They do have nutrients and vitamins in, so one every once in a while would be okay, but not like two per day.
Make a rule when you are watching tv, when the ad break comes on you have to get up and do something active like running on the spot or something.
Also try and plan what you are going to eat the night before and stick to it, you'll find you're less likely to sneak in extra things.