instead of just bashing Adsense with most of my post here on this forum I'm going to give away an Adsense tip i learned.
If you have an adsense custom search box place a link unit under of it.
check out how Ezine articles does it: [ezinearticles.com
I refer to it as suggested search term.
someone goes to your search bar to search to find more info, and what's there? Generic Key terms related to your sites theme "ie Adsense link units"
I would not recommend putting them above the fold. The click rate is super high, but remember link units clicks pays less.
when i first put them above the fold "in the same location as Ezine Articles" my ctr tripled, but the average CPC was 70% less, and my other ad units CTR dropped.
Also if you have a blog type of site, and have a related articles section, put a link unit in at the bottom.
Also, InfoLinks "Tag" works great here also.
All niches are different, so test it for yourself.