As more of us use rich snippets to give our web content value and visibility so it is that Google is rumoured to be much stricter on which authors receive coverage. Google Authorship rich snippets will only be shown for the highest quality sites.
The goal is to So keep aiming for high quality content, follow Google’s webmaster guidelines, and keep building your authorship authority. None of these approaches, of course, is a guarantee, but I expect it’s the best way to keep your authorship snippet intact.
Janet Driscoll Miller has created this useful info-graphic to help avoid some of the pitfalls of Google Authorship.