Besides al the tricks and books that others recommended, I love to: 1. Enumerate the limitations of my product with brutal honesty, that is obviously against my interest as seller. 2. Put clearly my interest as seller to make a sell so I tell to my customers that my opinions and advices are deeply flawed. So this way clearly I can lose a customer but almost always a I win a friend, and I always end with more info from my customers that my competitors and I close more difficult sales. Is not Advice that you find in the typical sales book but it gives you a lot of power over your customers