A bare minimum which every site owner should do, according to Rand FISHKIN:
That said, if you do nothing else, have a blog you update religiously every week (or every day if possible), occasionally targeting keyword phrases for SEO, and get accounts on Facebook/Twitter/Google+ where you share your posts. It doesn’t work for everyone, but it’s a content+social strategy that often yields consistent rewards. (source – Answers to 43 Questions About Search, Social, Content, Conversions and More | SEOmoz)
So, at a minimum, you should at least have a blog and promote it on social networks.