• Stay away from yes or no questions.
• Ask open ended questions that will help the person answering gain further understanding and clarity.
• Ask questions for which you do not have the answer.
• Ask questions that can help broaden the topic.

Here are some leads:

• Is there another aspect...?
• What are you still unsure about...?
• Can you say more about...?
• In your ___ paragraph you mention...?
• What did you mean by...?