The most widely researched form of influence is Social Influence and it's best known author / researcher is Robert Cialdini, Ph. D.
This Summary below was written by Robert and he permitted it's reprinting here.
"It is through the influence process that we generate and manage change.
As such, it is important for those wishing to create and sustain practical change to understand fully the workings of the this process.
Fortunately, a vast body of scientific evidence now exists on how, when, and why people say yes to influence attempts. From this formidable body of work, I have extracted six universal principles of influence--those that are so powerful that they generate desirable change in the widest range of circumstances."
Dr Cialdini explained these principles in his book and gave numerous examples.
In summary, these principles are:
• Reciprocation. People are more willing to comply with requests (for favors, services, information, concessions, etc.) from those who have provided such things first. For example, according to the American Disabled Veterans organization, mailing out a simple appeal for donations produces an 18% success rate; but, enclosing a small gift--personalized address labels--boosts the success rate to 35%
• Commitment/Consistency. People are more willing to be moved in a particular direction if they see it as consistent with an existing or recent commitment. Consider how small that commitment can be and still motivate change forcefully: Gorden Sinclair, a Chicago restaurant owner, was beset by the problem of no-shows—people who made table reservations but failed to appear and failed to call to cancel. He reduced the problem by first getting a small commitment. He instructed his receptionists to stop saying, "Please call if you change your plans" and to start saying, Will you call us if you change your plans?" The no-show rate dropped from 30% to 10% immediately.
• Authority. People are more willing to follow the directions or recommendations of a communicator to whom they attribute relevant authority or expertise. One study showed that 3 times as many pedestrians were willing to follow a man into traffic against the red light when he was merely dressed as an authority in a business suit and tie.
• Social Validation. People are more willing to take a recommended action if they see evidence that many others, especially similar others, are taking it. One researcher went door to door collecting for charity and carrying a list of others in the area who had already contributed. The longer the list, the more contributions it produced.
• Scarcity. People find objects and opportunities more attractive to the degree that they are scarce, rare, or dwindling in availability. Even information that is scarce is more effective. A beef importer in the US informed his customers (honestly) that, because of weather conditions in Australia, there was likely to be a shortage of Australian beef. His orders more than doubled. However, when he added (also honestly) that this information came from his company's exclusive contacts in the Australian National Weather Service, orders increased by 600%!
• Liking/Friendship. People prefer to say yes to those they know and like. For example, research done on Tupperware Home Demonstration parties shows that guests are 3 times more likely to purchase products because they like the party's hostess than because they like the products."
In his presentations, Professor Cialdini describes and emphasizes the ethical use of these principles. Only through its nonmanipulative use can the process be simultaneously effective, ethical, and enduring. And only in this fashion can it enhance a lasting sense of partnership between those involved in the exchange.
Robert B. Cialdini is Regents' professor of Psychology at Arizona State University in the United States.
So, the summary above refers to Social Influence, or factors that influence the masses.
BUT there are other forms of influence:
(and you can read about them by clicking the links below)
Since we are all individuals and have the capability of perceiving the same communication in totally different ways we can exert more influence if we understand Personal Influence and modify what we say depending on who we are talking to.
And, when it's used well, language affects us profoundly. That's why Rudyard Kipling said " words are the most powerful drug used by mankind". I call this Linguistic Influence.