Actually many publisher Know this tips & trick to optimize ads, but sometimes imprudent to do occasionally :)

Zone size & placement is one tips and trick which i got from Adbrite

The size and placement of the ads have a big impact on earnings.

Reach your visitors wherever you think they’re most engaged on your site. For instance, a prominent banner ad may generate significantly more earnings than low-placed ads. Banner zones—which enable banner or text ads—typically yield more revenue than text-only zones. On your own site, move the ad code to the appropriate place in the HTML. See below.

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