So you know those messages from Google in the Adsense dashboard that we all ignore? ;)
I saw one that mentioned one of my websites specifically in the subject, so I clicked on it.
It basically said, since you're maxed out on the number of ad units you have per page, why not try adding a link unit.
My first reaction was "because link units kinda suck". But after I stopped talking to the screen, I figured, what the heck, I'll give it a quick trial for a few days.
On my site I don't show ads to logged in members, I only show AdSense to "guests" who aren't logged in.
I added a 200x90 link unit with 4 links per unit in the morning and checked back the next day. The ad unit channel earned a few dollars. I didn't think too much of it because I thought "it's only a few bucks". Then I started using my 3rd grade math skills and multiplied that by 30 and realized that I just added a couple hundred dollars per month to my check.
Since it was a new link unit, I tried a couple different color combination over the next couple of days, but the first colorway ended up performing the best so I went back to what I originally tried.
I had tried link units in the past, but never stuck with them because they seemed like an unnatural think for a web surfer to do (coupled with the fact that they didn't seem to perform very well).
So the moral of the story is, if you have a high traffic site and you're only using ad units, it may be worth testing out a link unit to see if it gives you a little boost in your monthly PIP :)