For the longest time, I have had Adsense on the right-hand side of my webpage . . . above the fold. Last week, I followed Google's heat map suggestion and my clicks have tripled every day. I couldn't believe it. Granted, I hardly make $3.00 a day but this layout gives me incentive to keep building this page and trying to get more traffic to it. I only have one Ad unit on my front page and it sits right where the darkest orange is on the diagram of the heat map. Thought I would share this experience and here is their Heat Map diagram. https://www.google.com/adsense/support/bin/answer.py?answer=17954
I would ad a horizontal ad unit below the navigation bar. Also, be encouraged, $3 per day is almost $100 per month. Double your web presence and you can double your revenue. Repeat ten times.
The heat map is a nice guide, but I think it still really depends on the design and style of the site.... I try to keep it that at every point where a viewer may stop reading my site, there is an ad to send them somewhere else... kumiko xo