We own a website that gets, on the average, about 350,000 page views a day. We have been using plug and play code like AdSense and Exponential since we started. However, we are wondering if we can better monetize our websites by purchasing an ad server. We used DFP for a while but the income was less than the plug-n-play code so we went back to it. Anyway, what are your recommendations?

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you have to chose High Paying Keywords. Then publish Quality content. You know, quality content help you to rank your keyword easily. 
If you select High Paying keywords, so it will help you to increase your earning. Thanks

Allowing some outbound links to your website to improve your website content, without excess pagination, may also be a good option to monetize your website. Such outbound links are also likely to improve your website domain authority.