I have adult website for last 6 years and when I started my PR quickly grew up to 2 but then like 4 years it didn't grow up. Website grew pretty much but PR standing still. Any ideas for pushing it up to PR3 somehow?
I think google has preference to healthy website, though your website has large flow, it will not help upgrade PR.
My website jumped from PR0 (it's only 10 weeks old) to PR3 during the last update and all I did was blog commenting and forum posting, so I have to assume that backlinks play a major role as lifeplayer wrote above. I'm not sure if they need to be dofollow though, as I didn't pay any attention to that and just commented on any blog that had an interesting post.
nofollow backlink is not going to pass any PR juice to your website. only dofollow backlink will pass the PR juice, this is one of the main different between dofollow backlink and nofollow backlink
Just keep building quality backlinks and you wont have a problem, I can take sites to PR3/4 from brand new in a couple of months
why do you care about google PR - it hasn't had anything to do with traffic in a few years (or more)?