Link building is the best way to increase website traffic. Do Off-page Optimization like forum signature, blogs commenting, social bookmarking, Social Networking, Press Release and article marketing you can get more quality backlinks to your site and increase PR. Use all of these free techniques to increase traffic to your website.
Page rank is calculated based on the number of back links that you are getting. But its not just back links - its the Quality of back links. Google gives more value on the quality compared to those irrelevant sites that you are getting, so get back links on relevant and DoFollow sites.
PR is nothing at all to do with "quality" It is purely and simply the PR (real PR not the useless toolbar version) of the links pointing to any particular URI.