I worked for a rather large computer seller. After I had been there for a while, they initiated a new training program. Apparently, this is a very well studied method. It was called "Go for no."
Here's the premise of the policy:
A customer is almost 75% more likely to purchase something after saying no more than 5 times.
The new policy was to MAKE SURE the customer says no to EVERY thing you try to sell them AT LEAST 5 times. Needless to say, this resulted in many, many annoyed customers.