I have been serving up AdSense ads for those that arrive from the search engines (mostly Google), but as I'm broke and would like to raise the amount of money I'm earning from my weblogs, I just turned them on for any referrers. In other words, if you come to my site and browse around, you won't see the ads. The first time you arrive from a link on an external site, you'll see a big honkin' yellow ad on top of my post.
I'm going to watch that for a few days - if I see a drastic increase in the amount of money I'm earning after a week or so, I'll have to keep them. If not, I'll go back to the way they were. What I really need to do is get off my ass and add a flag in the DB I can flip for certain posts. For example, the Verizon Bluetooth post has gotten 11,000+ page views in the past month and has around 350 comments at the moment. There's little reason I'm not monetizing that some more, as most of the people commenting are not my regular readers and have found this site through online forums and links from other mobile sites.
By the way, the best way to wake up to lots of link love and comments? Post about Apple. I may become an Apple Blogger just to increase my ad revenues. ;-)