In a previous article, I talked about the importance of tweaking your Google Adsense adverts, specifically issues of size and placement. If you haven't read that yet, I suggest that you start by reading Improving Your Adsense Click-Thru before you proceed with this, the second part of the topic.
You're back now? Great. Let's keep talking about this interesting topic.
And before I go any further, please realize that while the specifics of this discussion are about Google adverts, the general user interface issues are true across the board: if you want someone to see something, you need to move it into their area of consciousness before you can have any success.
That's why it's so common to go to a Web site and stare blankly at the screen, trying to figure out where a given link is located, even though it's on a side navigational bar or similar. Even this site has that problem to some extent: quick, find the link that talks about custom 404 error pages!