Hi, To improve PR you need to have backlinks and if backlinks are from sites which has god PR can help you to earn PR without paying a penny.
In order to have good PR for your site, you need to have backlinks to your site from high PR sites. It is as simple as that.
To get PR of your site do good and related backlinks in High PR sites. Social bookmark plays good role for faster indexing. You should do backlinks as well as social bookmarking.
How do page rank and back links important to a site? Is it possible to monetize a site having no pr and low back links?