I'm trying to to change placement of adsense on a forum right now, and change from having the standard leaderboard at the top to having boxes more in the middle. Problem is fill, especially on the forums entry page. Looks like adsense doesn't find enough working keywords and phrases there. I don't know what kind of defaults I might use instead and to be honest I never really understood how to set up real defaults with adsense since all you've got is an image url it seems..... so was more wondering if there was any trick for getting keywords in for better fill. Looks pretty aweful with large empty boxes in the middle of a page.
I used to get alot of empty ad fills, especially on post pages. Switch my ads back to text_image and now they almost always at least pull a banner. Section targeting the post body will help as well.