A 'hands on' approach is necessary to make a site support itself through advertising.
I am currently recieving my best cpms from Burstmedia (about ($0.85 net cpm). As an experiment I once allowed all their available ads to run and recieved $0.11 net cpm. The same result would be true of any agency if they are not monitored and mananged.
The idea is to sign up and use a number of agencies. They all have some ads that pay well and a bunch that pay nearly nothing.
It is in everyone interest to deliver the best paying ads the first time they see a visitor. Afterwards they start sending lower paying ads until all you see are 'Free Zwinkys' and 'You won an Ipad' ads.
The idea then is to have a different agency follow up with it's best ad rather than having the first agency repeat the same ad or sending successively lower paying ads.