A common tactic: A salesperson who has completed a sale with you will ask for a list of friends, and call them up and talk about how they helped you and mention you passed their names on.
The world’s best car salesman (in Guinness Book of World Records) attributed his secret to two things:
- Offer a fair price
- Match the customer with a salesperson they’ll like 
This is a no-brainer: It has a strong impact, primarily due to the halo effect. It impacts:
- Judicial sentencing
- Jury payouts
- Likelihood of receiving help
- Persuasive power
- School punishments
The bottom line: Groom yourself and be well dressed!
We like people who are similar to us. This could be due to their opinions, religion, race, nationality, personality, interests, hobbies, profession, etc.
We are more likely to help those who dress like us.
Be wary of salespeople who are just like you!
That car salesman would send a greeting card very often which had the phrase “I like you.”
People may detect attempts at flattery, but it even works well in those cases! 
The compliments need not be true! False ones are as effective.
Contact and Cooperation
A familiarity with a well known surname helps in elections.
There was a theory that contact increases liking. But desegregated schools often ended up with more racial prejudice than before. Why? Because kids don’t like school (for many reasons) and more contact in a bad environment increases hostility.
A well known experiment in a boy’s summer camp: Simply separating kids into two camps resulted in an “us” vs “them” mentality.  Rivalry shot up.
Bringing the two groups together for social events (movies, etc) did not help. This is similar to desegregation in school. What did work? A situation where competition would hurt both parties, but cooperation would help both. The results were not instantaneous, but after a number of these manufactured “crises”, the divisions went away.
So familiarity due to contact works to increase liking, but not when that contact has distasteful elements in it.
The good cop/bad cop routine is another example: The good cop acts as if he’s on your team, so you start liking him. The contrast principle works here as well.
Conditioning and Association
We naturally dislike bearers of bad news, even though they are just the messenger.
Using celebrities in ads or for elections is part of liking.
Fund raising dinners: Make the requests once people have started eating. People give more with food than without. In general, just get people in a good mood or environment before making your request. Reciprocity also plays a role here.
We try to associate ourselves with successful people (winning teams, etc), using the pronoun “we”. But we distinguish from failures (“they”). We do this much more when we have low self-esteem at that moment (e.g. right after performing poorly in a test).
We may inflate our connections to successful people: Name dropping and rock band groupies are examples. Or we inflate the success of people we are strongly connected to (e.g. wife’s boasting about husband’s successes). In fact, sometimes wives get much more upset at the committee that the husband didn’t get an award than they are.
How To Say No
It’s OK to like something or someone. Just don’t let the halo effect impact you. And don’t stop liking the person!
|||This should explain why non-white customers in the US usually get someone who matches their race.|
|||I imagine they tell themselves “I see their ploy, but it can’t hurt to talk to them”.|
|||Think of Jane Elliot’s famous brown eyes vs blue eyes lesson.|