Monitor the messages on any given topics that have already been posted before posting your message. Your question may have already been asked by another student (In which case you should follow the thread for the instructor/moderator response), or you may reconsider you response to a reading once you have considered what the others have to say on the topic
Listen and respond to your peers.
Remember that the discussion board is a public forum, its purpose is to facilitate insightful dialog.
Keep postings relevant to focus of the course and make sure you post in the correct board:
assigned discussion board topic and prompt (answer the question)
questions about assignments, lectures, readings
When quoting another person, edit out what is not directly applicable to your reply. This will save all of us time and ensure your message is read by more discussion board contributors.