To get a PR 5 and stay there is really, really hard. You have to contiously be working on how to keep your site marketed properly and retain as many links as possible every day.
That is true. I had a PR5 for a while. Lost a PR6 backlink, and it dropped to PR4 and has stayed there through several toolbar PR updates.
My website went online in February 2009. Since the recent update, it has PR4. This website has backlinks from blogs with PR7, PR6, PR4 and a forum with PR2. I understand your question, but it's very hard to answer. The only thing I suggest is build backlinks on high PR (5 or higher) sites, blogs, forums etc.
I think it all depends on Backlinking from High PR sites. My Site started 10 days back : www.tiktech.com is @ PR3 whereas another site having good Alexa is having only PR1.
It's not hard if you know what your doing. But why would you want to? PR really means nothing, unless your site flipping. People are funny.
My site has just gone from PR4 to PR5. It has been a PR4 for nearly 2 years! I would say just keep working on getting quality links.