|The stupid question is the question not asked|
You should use code tags (<c>like this</c>)around preformatted things like code and error messages, and paragraph breaks to start new paragraphs, like this:<p>
You can see a list of those (and more) at the bottom of the preview window.
<b>Note:</b> You can go back and fix (update) your message if you need to. However, if you <i>do</i> update your node, you should add a note to your node letting people know that you changed it. You do this so that if anyone replied to your original node, then people reading the thread can make sense of it. Otherwise some replies will look totally wrong.
Update: Added "update" information, as suggested by (the somewhat impolite) roboticus.
Update: Added example formatting tags for bold and italics just because...