2. Balance your post, concede to the other side a little. On occasion a succinct post won't have room for this, but anything more than 2 sentences has space for a little give and take (see what I did there?). At the very least, it helps people understand your position better, and helps them follow rule #1.
3. If you aren't sure about something, say so, and maybe ask a good question. Nothing moves conversation better than a good question, but too many people on slashdot are too concerned with showing off what they know to ask a question that shows how much they don't know. People will occasionally mod up good questions, especially when it prompts an insightful answer.
This somewhat violates rule #1, so I apologize.