This might have already been covered, but...
The one last week with the bloke who chose the green chair and a pen. Derren had a large envelope in his right hand which magically contained the photos of the chair and stuff. He and the bloke got mic stands with crocodile clips on them, to hang the pictures off of. Derren got two stands from off stage, with his right hand going completely out of view for a moment, allowing him to switch envelopes.
Now, I'm not pretending to be super at spotting the trick. I wondered where he got the envelope and rewinded the show to find out. What's important is that the camera cut to the other bloke at the crucial time (he had to get a stand too, though I don't think it even ended up being used). I think this is along the same lines of a camera trick, though not as strong. Rather than misdirecting us and getting us to watch the other side of the stage, they used editing so that we coudn't see it at all.
Fair point on what some people have said though; it's not the trick that's important, it's getting people to accept some clever explanation rather than just going "it's not real, it's on telly".