When you are ushered in to the room and asked to take a seat, stride purposefully round to the interviewer's chair and sit down.
Now, gesture impatiently at the remaining (interviewee's) chair and say: "we'll need to be quick about this as I have a Boardroom lunch meeting in 15. Tell me, in two sentences, why you think you are the right man for this job?"
At this point the interviewer should be staring at you, looking confused.
Glance angrily at your watch and say: "No, no, I'm sorry. You're going to have to think on your feet in this position and I just don't see any ability on your part to embrace change or adapt to unfamiliar territory."
Stand up (as if the interview is at an end) and motion towards the door, saying: "Please send in the next candidate on your way out."
This normally does the trick for me.
GL