Thinking on the spot may be the most challenging part of running a discussion. You are reflecting on what you've already done, connecting threads of an earlier discussion to what you'd like to accomplish, addressing students' concerns, validating their comments, and pretending to act like your normal, happy self.
Since it is the nature of teaching, thinking on the spot is unavoidable. But there are things you can do to limit the amount of spontaneous thinking you'll need to pull off.
Some strategies for thinking on the spot are: