I want to add a promotional message to most of my webpages on my site. However I'm concerned that as a lot of my pages are under 500 words (and there is a valid reason for this) that it will cause internal duplicate content issues. I've had this sort of thing before and I know it causes pages to drop out of the Google index.
How can I stop this promotional content being indexed?
I've seen people recommend ajax pulling the content from a page that had a robots.txt blocking Google?