For example, Audette defines a number of key components that define
a valuable link--with the top factors being relevance and context. Beyond those concrete characteristics, the most valuable links are often difficult to obtain, require a human element (two people
need to exchange ideas), and involve a significant investment (be it time spent creating content or designing the site itself, developing relationships with influencers, or even the cost of paying for
a link).
Audette also offers a list of link-building sources, including social media hubs (blogs and forums related to your niche), email lists and directories, as well as human-powered search engines like ChaCha, Mahalo and Bessed.