While I have some preferences on how to moderate a panel discussion, there is no ONE way. It all depends on the topic, the venue, the panelists, and of course, the moderator’s own personal style.

My best advice is to make ALL of these small choices on behalf of the audience and the promise stated in the program materials. Listen for more. 

I like to think of a panel discussion in chunks of time or "segments" and allocate the time appropriately.

Let's use an example from this sample script for a panel discussion where the panel is only 45 minutes long.