There was an article from Stephen Warnock which I wish I could find. He basically lost all his confidence until he started working with a sports phycologist, the long and the short of it was he ended up switching his mindset from playing to avoid the boo's (I e. Playing it safe, taking no risks) to playing for the cheers - he said he played the best football of his career whilst working with this guy.
Also, when he was at villa MON took each player into his office and told them how rubbish they were - presumably looking for a reaction. Warnock said he went from feeling 10 feet tall to feeling like a mouse. No doubt this approach would have worked for some players but everyone is different.
It's really interesting because I think it shows the best managers will be the ones who understand what makes each individual tick. This obviously also applies outside of football.