Yeah, I've run one or two in my time.
As for what works... it really depends on the individual discussion, including the topic it's on and the people who attend. So I can't tell you exactly what will work, but I can tell you a few things that I've found useful.
1) Make sure everybody gets a chance to talk. In the more crowded discussions, some people may feel too shy to talk, or may feel like they're being ignored, so as a facilitator, it's your job to minimise that.
2) Make sure everybody knows what the discussion is, and what it's not. You may get some people who try to make it all about them, or think it's a chance just to get/give advice, so you have to balance everyone wanting to give or get help with actualling ensuring it's a discussion, in that everyone participates. I've had quite a few people enter a discussion late, and then disturb it because they had no clue what the topic even was.
3) Try not to accidentally log yourself out.
I usually do that at least once during each discussion, which is annoying if there's no CMods or PAs on to make me gold again.
Other than that, just take things as they come. Most often, discussions are good and you should all learn a lot, and if you run into any problems there's usually a
CMod or
PA on, so you can always talk to them.
Good luck with your first discussion. Let me know how it goes.