Recently, I found this interesting article: 9 Myths and Facts about Google Adsense (URL removed - by the way, very interesting), and I have a question: Myth 7: Repeating visitors will stabilize your Google Adsense incomes. What you do, to "monetize" repeating visitors?
Repeat visitors tend to become 'ad blind' because they get used to the environment of the site. However, if the repeat visitors or on a blog, forum they will still be presented with different type of ads as the content will change.
I don't think you should focus on monetizing repeat visitors but instead leverage them to generate content. For example, I have a blog and a nice healthy vocal set of repeat visitors who I elicit for comments and on more than one occasion the SERPs have ranked me high for keywords that appeared only in the comments section of the blog. In that respect, I guess it is monetizing repeat traffic however indirectly.
I think Bargainr is 100% on the money with that.. I dont even show ads to my 'members' (they are most of my repeat visitors) on most pages; but THEY (return visitors/members) generate all (well, most) of my content now via posting comment and new forum threads. All this extra content feeds the Google Spider, which in turn brings in more traffic.. Once the snowball starts rolling, it gets bigger and bigger.