Before this end of July Google PR update, i believe that to achieve higher Google's PR i must work hard to build quantity and quality back links for my sites. But the fact is different with what i expect. 1 brand new site with unknown PR status that i just park it on Whypark now become PR 3 without any effort to build backlinks or submit it to seach engine. Meanwhile another sites established with PR 4 and 3 for at least 1 year, build backlinks anytime i have a chance (incl from .edu sites), stepped on by google become 2 and worst become 0. Anybody can help me to explain about this and how to avoid this PR decreased happen again in the next update?
Your site has been penalized by Google since your site's PR dropped from PR 2 to PR 0. You can correct this by reading the Google Webmaster Guidelines.
You can always avoid website penalization from Google. Read about it here. http://www.dailytechnologytips.com/2008/07/how-to-avoid-blog-or-website.html And about your domain got some PR without backlinks. That is possible with Google. Google give PR values to Domains as it grows in age. Not just backlinks are responsible for PR, there are other factors too.