Give honest and sincere appreciation. - Dale Carnegie
Lincoln once began a letter saying: "Everybody likes a compliment." William James said: "The deepest principle in human nature is the craving to be appreciated." He didn't speak, mind you, of the "wish" or the "desire" or the "longing" to be appreciated. He said the "craving" to be appreciated...Let's try to figure out the other person's good points. Then forget flattery. Give honest, sincere appreciation. Be "hearty in your [compliments] and lavish in your praise." and people will cherish your words and treasure them and repeat them over a lifetime - repeat them years after you have forgotten them.