Variant: You can make more friends in two months by being interested in them, than in two years by making them interested in you.
Source: How to Win Friends and Influence People (1936), p. 52 (in 1998 edition)
Quotes from work
How to Win Friends and Influence People

This fascinating book deals with what is probably the greatest personal problem in the world today: worry. It is a practical, concrete, easy-to-read and inspiring handbook on conquering worry. This book may easily double or even triple the happiness you have.
“If you want enemies, excel your friends; but if you want friends, let your friends excel you.”
How to Win Friends and Influence People
“Don't be afraid of enemies who attack you. Be afraid of the friends who flatter you.”
Source: How to Win Friends and Influence People
Source: How to Win Friends and Influence People
Variant: It isn't what you have, or who you are, or where you are, or what you are doing that makes you happy or unhappy. It is what you think about.
Source: How to Win Friends and Influence People
Source: How to Win Friends and Influence People
Source: How to Win Friends and Influence People
“By fighting you never get enough, but by yielding you get more than you expected.”
Source: How to Win Friends and Influence People
“Once I did bad and that I heard ever. Twice I did good, but that I heard never.”
Source: How to Win Friends and Influence People
“Talk to someone about themselves and they'll listen for hours.”
Source: How to Win Friends and Influence People
Source: How to Win Friends and Influence People
Variant: you can measure the size of a person by what makes him or her angry
Source: How to Win Friends and Influence People
Source: How to Win Friends and Influence People
Variant: When dealing with people, let us remember we are not dealing with creatures of logic. We are dealing with creatures of emotion, creatures bristling with prejudices and motivated by pride and vanity.
Source: How to Win Friends and Influence People
“A man convinced against his will
Is of the same opinion still”
Source: How to Win Friends and Influence People