"yeah...yeah....really? shut up! hm... yep... oh...! hey, a rat with a tie behind you!" all about it is timing now: leave at the exact moment at which they turn around. till they get what u just sed, you'll have disapparated.
I do one of the following things: A) Look at them like I wanna kill them and they usually walk away. B) Change the topic myself and make it more interesting for me. C) I would start to close my eyes and shut that person out and/or fall asleep. D) I say I need to go to the bathroom and never come back. (I swear the next one will be my last one) E) I do the sensible thing and just tell them gently they are really boring and just walk away