Well than i am very curious.. I get around 2.5-3K Clicks a day.. Didn't know that premium exist.. Anyways, im not premium with my 3K clicks a day lol. Will let you know if it ever happens..
I think Google also goes with the brand name. For example if many people search for your website name in Google, they'll think you have a brand people like and are more willing to give such people a premium account (because their ads appear on good sites that way etc.)
I've heard rumours that you need around 10M pv's a month per website to make premium. That is what I have heard anyway, I could be wrong.