Simple. Just select the ads with a higher eCPM and unselect the ones with a lower eCPM. If you have a high CTR then also include the ads that pay per click. I normally have all the ads that pay per click and then all of the CPM ads that pay over $0.40 CPM. I don't get a 100% fill rate, but I have a chain of networks so they others will help to fill it.
I guess it can't be as simple as only selecting the high eCPM ones or everyone would do it. I won't get so high a fill rate. Do higher traffic sites get more of a showing of the high eCPM ones?