Link Building is the only factor of maintaining page rank in search engine and these factors are off-site in that they are not controlled by you or the coding on your page. It includes only page rank and link popularity.

To increase link popularity and ranking of website in search engine, we can follow some link building techniques trending now-a-days-

Social Bookmarking
Some Social Bookmarking Sites - delicious.com, StumbleUpon.com, Digg.com, slashdot.org, reddit.com.

Social Media Engagement
Some High PR Social Networking Sites - FaceBook.com, Twitter.com, Pinterest.com, Linkedin.com, Plus.Google.com.

Link building on particular niche sites
Sites category based on particular business website’s category.

Forum Engagement
Category of forum boards based on business website’s category.

Content Sharing
Some content sharing websites - Tumblr.com, Wordpress.com, Sites.Google.com, Joomla.com, Quora.com.

Document Sharing
Some document sharing websites - issuu.com, slideshare.net, scribd.com.

Video Sharing
Some video sharing websites - YouTube.com, Vimeo.com, MetaCafe.com, DailyMotion.com.

Press Release Submission
Some press release submission sites - PRLog.org, PR.com, Newswire.com, OnlinePRNews.com.