Psychology of influence
In 1984 American psychologist Robert Chaldini published a book "Psychology of influence", in which he posed 5 principles of the conviction. In this title we'll consider and discuss each of them, will try to understand why they have stood testes of time. 
Important to know, these methods are not used to manipulate by people, it must be sincere and based on the friendship sympathy. Greedy people always loose in a long perspective and spoil their reputation.
6 principles:
1) Reciprocity.
 Give something . Through time, ask for something in return, but not equivalently. Be sure, people know what they get. Tell them about your sincere intention to help. The price and amount of your efforts must be low, but high for your interlocutor.
2)Commitments and sequence.
 We also must conform our actions and promises with our convictions and values. When we promise, we must  do everything to keep them and feel responsibility. When we do something that don't correspondent to the convictions and values, so we can change these internal aspects to become consistent. 
We can justify out commitments devising new rationales to be sure in our right choose.
3)Essence
Show people how the others do those things you need from them. We follow people's actions when we are not sure in our experience and knowledge. 
4)Principe of power and authority. 
If we notice professionalism in someone and this person affirms something as genuine, so we will trust him. Also we are submitted to those people who have strongly marked tongue of body. We have been brought and are used to listen to the teachers, parents, bosses at work.
5)Principe of sympathy
We are tend to sympathize to nice people reasonably. Be friendly, show other people that you like them, be interested in their opinion frankly. Two things strengthen sympathy: similarity and praise. If you show that a person looks like you and make him a compliment, it will change his mind. 
6)Principe of deficit
We want to have that things ,maybe we won't able to get in the future. When the things become less available, they become more desirable for us. If we have a chance to get it now or in the future, we prefer now. But it is not a fact that we need this thing or service.