Cialdini 's Six Principles of Influence

As humans, we generally aim to return favors, pay back debts, and treat others as they treat us. According to the idea of reciprocity, this can lead us to feel obliged to offer concessions or discounts to others if they have offered them to us. This is because we're uncomfortable with feeling indebted to them.
For example, if a colleague helps you when you're busy with a project, you might feel obliged to support her ideas for improving team processes. You might decide to buy more from a supplier if they have offered you an aggressive discount. Or, you might give money to a charity fundraiser who has given you a flower in the street.

Cialdini says that we have a deep desire to be consistent. For this reason, once we've committed to something, we're then more inclined to go through with it.
For instance, you'd probably be more likely to support a colleague's project proposal if you had shown interest when he first talked to you about his ideas.

This principle relies on people's sense of "safety in numbers."
For example, we're more likely to work late if others in our team are doing the same, put a tip in a jar if it already contains money, or eat in a restaurant if it's busy. Here, we're assuming that if lots of other people are doing something, then it must be OK.
We're particularly susceptible to this principle when we're feeling uncertain, and we're even more likely to be influenced if the people we see seem to be similar to us. That's why commercials often use moms, not celebrities, to advertise household products.

Cialdini says that we're more likely to be influenced by people we like. Likability comes in many forms – people might be similar or familiar to us, they might give us compliments, or we may just simply trust them.
Companies that use sales agents from within the community employ this principle with huge success. People are more likely to buy from people like themselves, from friends, and from people they know and respect.

We feel a sense of duty or obligation to people in positions of authority. This is why advertisers of pharmaceutical products employ doctors to front their campaigns, and why most of us will do most things that our manager requests.
Job titles, uniforms, and even accessories like cars or gadgets can lend an air of authority, and can persuade us to accept what these people say.

This principle says that things are more attractive when their availability is limited, or when we stand to lose the opportunity to acquire them on favorable terms.
For instance, we might buy something immediately if we're told that it's the last one, or that a special offer will soon expire.