When writing text for others to read, try and write directly to your audience.
Explain your ideas as clearly and simply as you can to help avoid misunderstandings. One thing you can do is to avoid long words when a short one will do.
In the forums it will really help to keep your whole posts short and on-topic. Instead of one long rambling post that makes many different points, it may be better to write several shorter ones (they may even belong in separate forums).
Re-edit your text as much as necessary until you get it right. Even in forum posts you have 15 minutes after you've posted it to go back and improve it if necessary.
When replying to others, try and think of interesting questions you can ask them. This will help both you and the other person think (and learn!) about the subject you are discussing.
More info about asking questions