* Comment when you have something to contribute. The contribution can be small, even just adding your own experience. A "me too" comment will likely get downvoted. A "Me too. I often experience/feel this when..." will likely be left neutral or upvoted, depending on topic and content.
* Assume the other party in a discussion or argument is acting in good faith. Try to look for alternate interpretations for offensive or upsetting comments, and ask, if at all ambiguous. Even if it's clear the opinion is at odds with your own, don't condemn someone outright, ask why (and try to do so in a way that doesn't sound like you're setting them up).
* Tangents are okay. Responding to a particular comment but addressing the conversation is okay. Try to make it obvious when you are not addressing the person you replied to specifically. Context matters, especially for those that see the reply, and it's easy for them to assume it's supposed to be meant as a rebuttal to them when they've been seeing lots of rebuttals.
I just wanted to point out that in my case, getting points was not that easy. But I actually don't care about it, so everything is fine for me. :)
But then, I don't comment often and I don't care about being able to downvote, so I think I'll survive.