One thing that's been a huge time-saver for me: scan the discussion first, and if someone else has already made the point you're thinking of (even if they made it badly), refrain.

Only contribute when you can be adding something to the conversation. View your part as just that, a contribution - something that you would be proud to see published somewhere "official". Imagine someone coming along and summarizing the debate for a publication.

When in doubt, disengage.