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Good practice, good content, inbound links and you'll climb slowly.
I've had a number of PR 2 pages sitting in google's index for some time. They don't have many links, hence the pr of 2, and they aren't likely to garner many, if any, new links. While the pages are not important to the masses, the pages are important to some, so the pages remain.
I do not really pay attention to page rank now anyways. I have pages that rank in top 10 serps that do not even have backlinks.
I concentrate on good content and valid html now. Works for me....
And even if your site is old as has high PR on main page, there will always be deeper pages with PR3, PR2 and lower. It's absolutely normal.
Do you really think that valid code is important? cnn.com can't validate, microsoft.com can't validate, an dI could go on forever listing top sites which doesn't seems to care about HTML validation.
And if the rule "design for your users, not for search engines" is valid, then we can say that if browser can show it, then th code is good.
Just my personal, non professional, opinion