Many members are still telling that on-page factors can boost PR, but there's no direct relationship between on-page factors & Google PR. Google PR can be improved only through link building from high PR pages. I must explain that how on-page factors can be indirectly help in boosting Google PR. By creating a well designed website with proper on-page optimization & high quality contents, then high PR reputed sites will naturally give back links to the site & thereby the Google PR of the site will be improved. So Google PR is not directly improved through on-site optimization.