I received 40k / day and had a PR0 and still I got most of my traffic from google so like Crook said PR doesn't mean much these days.

Plus your comparison is kinda weird: 10k / day doesn't mean your should have a high PR or when having a high PR mean you'll get more traffic (don't get me wrong: it used to mean you're getting more traffic from google but see my PR0 experience above)

To get back to your question it gets higher when you trade quality links and with the same or higher PR or get simple linkbacks by adding your link to other forums (forums who do allow this; no need to spam or get banned for a simple linkback)
Inevitable Reviewed by Inevitable on . 10k adult site still pagerank 1? what the heck, my adult site which gets 10k hits a day for a year now, has still pagerank 1? never trade backlinks, but can it not get higher himself? Rating: 5