First, begin with a salutation and their name; this shows who you are speaking to and will help you remember to address their post directly.
Second, chunk your supporting information into paragraphs of 1-3 sentences. This will make it easy for everyone to read.
Third, be kind. You can disagree. Disagree is great for a discussion, but be respectful. We do not want any petty arguments!
Fourth, end your response with a substantive question to you peer. Put it in its own paragraph and in Bold