You need about 20 million page views a month to qualify. You then have the option to customize your ad format - which must be approved by adsense.
Not sure 100%, but I think you have to be a really good performer to become a premium publisher, ie. a very large publisher.
Not sure you need 20 million, I hit premium status approx 5 million page views ... to be honest I am now making less than I was. I used to be able to prime the ads with a "google_hints" tag and tell them what ads to display. Now this option is not available ... yet mind you the economy is taking a beating right now, so it could be too early to tell.