This article is broken up into two sections: sites and forums. Obviously, placing ads on forums is a little different than placing them on sites so I decided to split them up.
Site Placement
     When placing ads on a site, you have to consider where people are looking the most.  Generally they're looking at your content, which is usually in the middle of the page.  This is why text wrapped around ads is especially effective, because the user is reading the text and is likely to read the ad as well.  Google Adsense has released a heatmap of where the best places are to place ads on your site, and where people look the most.  You can view it here:
Site Heatmap Link



Forum Placement
     Although new users to your forum may take a look at the header of your board, regular posters skip down to the posts and content, which is why placing ads after the first or second post can be especially effective.  In addition to this placement, a sidebar ad can be effective, but avoid any ad at the bottom or in the header.  They will generally go un-noticed.  Google Adsense also released a heatmap for forums.  Check it out here:
Forum Heatmap Link