They all have these nice looking banners and ad images strategically placed in sidebars and headers. Then they have their ads wrapped in text in the content, but they forget about actual embedded content text links.
For example, let's say it is a Nikon Camera Model X review site. If you have 5 300 word reviews then there are a ton of opportunities there. Every single mention of "Nikon Model X" should be a hyperlink to that products sale page. Also think of the other products you can throw in there. "I found that "camera cleaner x" works great on my "Nikon Model X". "Camera cleaner x" can be a hyperlink to that products sales page, now you have suddenly diversified your sites effectiveness.
I know many people are saying "duh" we know this, but this is for the many people that forget about it when building sites, believe me there are quite a few.