If you can read it to another person before posting it, even better. Although electronic communication is still young, many conventions have already been established. This is regarded as shouting and is out of place in a classroom. But Web-based courses have some added constraints not present in other arenas. Treat your instructor and your fellow students with respect.
Brevity is best.
Be as concise as possible when contributing to a discussion. Web-based courses require a lot of reading, and your points might be missed if hidden in a flood of text. If you have several points that you want to make, it might be a good idea to post them individually, in several more focused messages, rather than as a single, lengthy, all-encompassing message. Stick to the point. Contributions to a discussion should have a clear subject header, and you need to stick to the subject.