Lots of hard work, and a billion+ pageviews 
I am without a doubt the top individual adsense publisher in terms of pageviews. Me talking about how I did it won't help you, but here is what i found you need to be successful in adsense.
1. Get a database of IP's so you know where your traffic is coming from. Then create channels for each country. Its not uncommon to see US traffic with a CPM of $5.00 and a CDN traffic at 20 cents and vice versa. If you have access to the hints option, give different hints based on IP. ie if your page is about 401k plans, that won't get you anything outside of the USA.
2. You have to create sites that will bring in repeat traffic. If you think you will get rich off SEO think again. If you create a Free jobs site you could net 30 million + a year if you got big. Club listings site, free religious personals etc would all be big money makers. Look for established markets and offer a service for free and support it with adsense.
3. Have your users create content and lots of it. User reviews of night clubs, Resorts, golf courses etc. Build your site around your users and make them part of your site, don't build your site for consumption.
4. Do not enter markets with a lot of competition monitized via adsense. Try and undercut paid content markets by offering a free service, or better yet create your own market.
5. Keep your site dead simple, it has to load fast and have no more then 2 ads and 1 or 2 pictures other then your logo. Do not confuse your user, give them what they want and give it to them fast.
6. Troll around various forums and if people are not talking about your market, there is a good chance you will make money.
