Well, the PR on the toolbar is just that - Toolbar PR. The real Google PageRank mechanism is something contained internally within Google, and not shared outside of Google. Anyway, the way the real PR or the Toolbar PR is developed is also a big secret!
You need to get links with high PR value from other websites linking to your website and they pass the PR value not using nofollow tag, then on the next update you will get PR value, depend on the quality of your backlinks and how many links on the linking url.
I wonder how everything started, then PR thing started all sites had 0 pr... I was reading somewhere that new site without any links to it has 0,85 PR.