Dale Carnegie tells us exactly how.
1. Begin with praise and honest appreciation.
2. Call attention to people's mistakes indirectly.
3. Talk about your own mistakes before criticizing the other person.
4. Ask questions instead of giving direct orders.
5. Let the other person save face.
6. Praise the slightest improvement and praise every improvement.
7. Give the other person a fine reputation to live up to.
8. Use encouragement.
9. Make the fault easy to correct.
10. Make the other person happy about doing the thing you suggest.
Dale Carnegie (1888-1955)