Whether you’re a teacher or sales professional, you will not achieve success unless you believe in yourself, the organization you work for, and the product you are selling.

The two professions are closely intertwined—as Jack Malcolm, sales trainer, rightly realized when he said, “Good salespeople do a lot of teaching, and good teachers do a lot of selling. Both professionals only succeed if their customers or pupils do—you can’t be a teacher unless someone learns, and you definitely can’t sell without someone buying.”

In this...