Who do you really need to persuade?

When you are selling, sometimes you need not only persuade the person you are selling to, but you need to persuade her parents, sister and friends. Many times, when a person is about to buy a new product or a service, or maybe even attend a university (like the college where I work), that person seeks advice from people she trusts. The problem is that we never know who seeks advice, and we never know when people seeks advice. Some people might talk to their friends before buying a new book. Some people might ask their dad before they are going to apply for a university, some people might ask their sister when she is going to buy a new dress. Many people might never ask at all. The only way we can be sure that our marketing works, is if we can persuade them all.