Getting rid of a 'double chin' is a matter of exercising more and watching what you eat. You cannot do what is called spot reduction. This means that the body will burn fat as a whole, not in one specific area that you are focusing on. Since weight loss (or losing body fat) is a matter of burning more calories than you consume, you are better off doing lunges to get rid of a double chin than any kind of neck exercise or infomercial contraption.
Focus on adhering to a well-rounded strength training and cardio program and watch what you eat. I highly recommend tracking your food using online software. It helps to see the amount of food you consume each day.