Begin with praise and honest appreciation.
Call attention to other people's mistakes indirectly.
Talk about your own mistakes first.
Ask questions instead of directly giving orders.
Let the other person save face.
Praise every improvement.
Give them a fine reputation to live up to.
Encourage them by making their faults seem easy to correct.
Make the other person happy about doing what you suggest.
- Manager vs Leader: http://www.appleseeds.org/Manager-Leader_Bennis.htm