Marking Up Acronym and Abbreviations

How often should you mark up the same acronym if it appears a lot on a page?
Every time or just the first occurence?

I have a page with XHTML in it 30 times, if I mark it up every time it looks
a bit too much with the underlines and it will also increase the page size a
fair amount.

Nigel

MIS Web Design
http://www.miswebdesign.com/