Appealing To The Vanity In Others

THE PEASANT AND THE APPLE-TREE
A peasant had in his garden an apple-tree, which bore no fruit, but only served as perch for the sparrows and grasshoppers. He resolved to cut it down,and, taking his ax in hand, made a bold stroke at its roots. The grasshoppers and sparrows entreated him not to cut down the tree that sheltered them, but to spare it, and they would sing to him and lighten his labors. He paid no attention to their request, but gave the tree a second and a third blow with his ax. When he reached the hollow of the tree, he found a hive full of honey. Having tasted the honeycomb, he threw down his ax, and, looking on the tree as sacred, took great care of it, (FABLES, AESOP, SIXTH CENTURY B.C).
If you need to get someone to do your bidding or perhaps do you a favour, it is wise that you appeal to his/her self-interest, for it is that alone which moves some men.

The truth about the world as we know it, is, there are very few people who genuinely and without need for reciprocation, give you what you ask. It is not an entirely negative trait, but it is a trait nonetheless. A lot of people at one time or the other may have come across a person whom they needed assistance from, but somewhere during that discussion, person A had to offer something in return to person B, so as to speed the decision process evidently going on in person B's head. Yes, we have all been there.
The trick in being successful at this though requires some kind of subtlety, you don't want to seem too manipulative, else you might end up getting the opposite of what you planned. Do not bother reminding him of your past assistance and good deeds, he/she will just find a way to ignore you. People naturally do not like being indebted to others, the reason for this mostly is because in such a situation, they feel like they don't have a choice. Instead of reminding said helper about what good deed you did before, uncover something in your request, something that will benefit him and emphasize it. Seven out of ten percent of the time, this strategy works.

(Most men are thoroughly subjective that nothing really interests them but themselves. They always think of their own case as soon as ever any remark is made, and their whole attention is engrossed and absorbed by the merest chance reference to anything which affects them personally, be it never so remote. ARTHUR SCHOPENHAUER, 1788-1860)

QUICK NOTE: A lot of people will deny they ever required something in return for the good deeds or assistance they rendered someone else. Well, don't expect otherwise. Nobody ever admits to serving their own personal interests.