How do convincing speakers vary their speech when they speak for different purposes and to different people?

They vary their speech according to the audience and what their intentions are. They'd target the audience's soft spot, you'd base them on their age stereotypes. For example, teens want whats 'in,' whats new and what would give them that temporary satisfaction. Adults want high quality things, things that would last them a long time. Seniors.. Well, they want whatever they need. And I emphasize on the need. Salesman would mainly start advertising, obviously.