- Taken From Adsense Confession Books
#1 - Not using Channels

I didn´t make this Adsense error but if you aren't using Adsense
Channels you are losing money and don´t have a way of tracking
your development. If you don't believe me, set up at least URL
channels for your pages and you'll see in real time what works and
what does not.

#2 - Using the horizontal format

So far, the results are much better when people use the vertical
format.

#3 - Placing Adsense Ad Units on the top, bottom, right and
left of content

The Adsense publisher community may not totally agree, but don't
hide your ads by neatly stuffing them on the borders of your site.
Put your Adsense Ad Unit in the middle where your visitor will be
forced to see them. It's not that your visitor doesn't want to see the
ads it's that over the years of web surfing we've become blind to
traditional banner locations.

#4 - Adding your sites to the filter out list

One Webmaster thought that the way to let Google know that you
wanted to add additional sites to your AdSense account was to list
the URLs under the Advanced options tab. In reality, this tab is used
to list the sites you want to filter out (i.e., not display AdWords) on
your site. Google automatically knows what pages you want to
include under your AdSense account when you add the code to the
pages.
So, you can add any site you own without additional paperwork.

#5 - Not testing.

Because of the limited information Google currently supplies, this can
be difficult. Nonetheless, you want to see what works and what
doesn’t on your site. Experiment.

#6 - Applying too early.

If you don’t have at least 20 good content pages and some traffic,
you’ll most likely be rejected.