Chat with ZC this morning. She has three points on a good salesman: 1) likability, 2) thick skin, and 3) creditor mentality.
Of the three, the first is about personality. It is effective and easy to work out (see the evolution of Ms. Hillary Clinton). The thick skin implies that you must prioritize your goal over your feelings: do whatever it takes. The last point is about the belief: trick yourself to believe before selling.