External links of incoming links to your site are the most important factor in search engine optimization. Good quality links with keywords in anchor text, especially from relevant sites,  will result in high rankings for your web site. Engaging  a professional SEO company will help in getting a good number of quality links.

Build great content or ‘authority’ content in your vertical or domain and you will get links from relevant sites automatically or on request is recommended by Google and SEO gurus, but is easier said than done.

Submitting good keyword rich articles to Article Directories with high Pagerank is one great way to get links.  But make sure you 'spin' your article for submitting to multiple directories.