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My main site moved to a PR7 after the last update mostly based on a great link on a PR8 site and a few strong PR6 and PR7 links.
How did I get those links? It wasn't by sending out hundreds of emails asking for people to link to me. Instead, I've been focusing on creating quality/differentiated content and those sites found mine and linked on their own.
My improvement to a PR7 hasn't had a big effect on my rankings for my main keywords, but it has improved my rankings on other keywords I hadn't been focusing on. I'm now building optimized pages around those keywords. My higher PR is a help in competing, but it means nothing on its own.
Research keywords - build quality content - study your logs - build quality content - rinse - repeat. PR increases will be a natural consequence of the process.
But it can change -- my index page went from 6 to 7 and back to 6 again over a few months.