My favorite trick is to have a salesman tell me that there is a better deal somewhere else. Anyone who says 'the offer is only good until you walk out the door!' knows that once you walk out the door you'll find something better.

Your buying <thing> once. The salesman sells <thing> every day. Whenever I'm given an exploding offer, I hear an expert in the <thing> market telling me that there is a better deal to be found.