If I may add something to (2) (and (3), too) I'd say to be concise and not wander too far: keep the focus and don't waste time. Moreover I think that a contribution, let it be a comment or a main argument, is much more interesting if it's different from what it has been already said before. I often hear people asking questions or offering arguments that have shown up before, without providing any new insight. Basically you could say "Listen before you speak".