Although theses days the search engines are also taking into account relevancy in their algorithms, where as in the past having your link appear on ‘any’ site would have been satisfactory.
Along with solid on-site optimization, more credibility is given to websites whose content is of a similar theme as well as the page rank of the site that is linking to you. These other sites consider your site important enough to link to. So, in the engine's view, your site is considered important also. Once a website has been optimized, getting related links to the site is 95% of the Internet Advertising work. The work of obtaining links is never done. If we stop getting links to a site, the links get stale, and your competitors may pass you.
However, one of the most difficult areas of SEO is building link popularity, due to the fact that search engines do not want "artificially created" links. The days of link farms and huge link exchange programs are over. Rather, the engines want links from authoritative sites, or links from sites that share the same subject matter as your site.
We have established techniques to gain relevant permanently placed links on pages indexed by Google offering one-way or exchange links.