Also important for SEO are the so called “off-page” strategies. Rather than examining just the webpage itself, modern search engines take into account other factors such as the number of links to a page. The more inbound links to a web page the higher it will rank in the search engines.
Guest blogging (publishing work on the sites and blogs of others) is one method of link building that is safe and effective.
Building links can also be accomplished by sharing content across social media such as Facebook, LinkedIn, Twitter, Google+, Pinterest and YouTube. Content that has been extensively shared on social media is taken into consideration by search engines as an indication that it is of higher quality.
Do not pay for links! This kind of practice can get your site banned from Google and other search engines.
Learn more about current SEO practices for building quality websites with this in-depth guide to Google ranking factors (Search Engine Watch).