How can I break my 19 month old's habit of biting?

Say no! To help him grow quickly through the biting stage, when you observe biting, say, "no! " firmly, so he can tell you are not pleased, then repeat and name the behavior, "no biting! ". Then scoop him up from the situation and set a timer for about two minutes (the rule of thumb is one minute for every year he is old) and pay no attention to him for the next two minutes, even if he acts sorry or angry.