You have to be on your feet in thinking about what the other person will say. Normally the fight is about price and what you get for the price. If you know your stuff, you can group several parts of the offer and create a new price that might not be calculated by the other person, and you might get a good deal.
You also have to understand when you are buying. If there are only three left and 30 people begging to buy, move fast. However, if there are 30 left and 3 people, maybe you can make your money do the talking.