1) Using same anchor text
It has been found that most of the websites that were affected had high anchor text ratios. Statistics says that website that had more than 60 percent of the same anchor text have suffered from penguin update and website that had diversified anchor text were least affected.
2) Publishing crappy content
This is one of interesting observation post-penguin. It is said that Google is trying to penalize the production of crappy content. So If you are producing lot of content for your website which barely brings any traffic and not useful to users then stop doing it.