I get nothing from Adsense, so this may be a result of my small sample, but a strange thing. I've experimented with all the obvious - leaderboard below the logo, ad unit in upper left above the navigation and so forth. Surprisingly, again and again, I ge the most clicks from a tower that's way down below the fold on the left below almost everything useful. In other words, only the users who are actually scrolling pretty far down see it. I'll balance it with a banner at the top and something on the top right and yet, there it is. Against all expecation, my typical user seems to enjoyr scrolling first, clicking later.