I think it's easy to overestimate his skills; I remember a number of occasions where, whilst watching, I assumed psychological brilliance was the only possibility, yet later thought of a much simpler hypothesis. I'll list one example here.

In one trick, which he did on his stage show Enigma, he would have a lot of audience members write a celebritiy's name on a piece of paper and fill a basket; a randomly chosen audience member selected a piece of paper and Derren, from holding the man's hand, painted a picture of the celebrity's face. He makes a good deal of showmanship from the contact with the man's arm, and appeared to be having extreme difficulty - the face looked nothing like a face! Finally he turned it upside down and there was Elton John. For a long time I believed he was a psychological genius, before I realised that somehow forcing the celebrity's name, even in front of the audience looking at the empty bucket being filled and then handed to the audience member, was a far simpler hypothesis.