You should define an acronym whenever you use it, or at least once per post.
How to do it
The first time you use an acronym, mark it up with an tag, like this: The first time you use an acronym, mark it up with an<acronym>tag, like this:
<acronym title="cascading style sheets">CSS</acronym>
This item is hot on daypop and no wonder. Mark has created profiles of users who would benefit. Lovely, see Bill’s story for instance.