9 Ways to change people without giving offense or arousing Resentment

Rule 1: Begin with praise and honest appreciation.
Rule 2: Call attention to people's mistakes indirectly.
Rule 3: Talk about your own mistakes before criticizing the other person.
Rule 4: Ask questions instead of giving direct orders.
Rule 5: Let the other person save face.
Rule 6: Praise the slightest improvement and praise every improvement. Be "hearty in your approbation and lavish in your praise."
Rule 7: Give the other person a fine reputation to live up to.
Rule 8: Use encouragement. Make the fault seem easy to correct.
Rule 9: Make the other person happy about doing the thing you suggest.

Source: How to Win Friends and Influence People