Mark Suster and Fred Wilson have both posted on the topic of being a good panelist. I'll throw in the best advice I ever got on the topic (from Alex, who got it from someone else):
Disagree with other panelists.
A panel of people agreeing with each other is generally tremendously boring, and any topic that's interesting enough to be brought up as a question to panel must have at least two sides that can be argued.
So argue the other side. Find someone else on the panel willing to go toe to toe with you, and disagree with what they say. The point is not to start a fight, but to create an engaging discussion. It'll force you to think on your feet and it'll entertain the audience.