1) Rules of the classroom apply here also.
2) Would you say it to the person's face? Think before you speak. Be respectful.
3) Make yourself look good online. Always edit.
4) Share expert knowledge. Know what you are talking about. Always link to your source.
5) Respect other people's privacy. Never share personal information. If necessary, leave out the person's name.
6) Be forgiving of other people's mistakes. We all make them.