Okay, so you've done your SEO work, gotten lots of incoming links, traffic is swelling, and your PageRank is high enough that even your grandmother realizes things are going well. Now what? Sure, you'll sell more products and make more off AdSense. But that's not all you can do with it.
I finally took a few minutes out of my busy schedule a few days ago and put together sponsorship programs for two of my websites: Chess Hounds (an online chess site) and Chordata (an RSS & Atom feed directory). Traffic is healthy on both sites. The Chess Hounds homepage has PR7, and some of the interior pages have PR6. The Chordata homepage has PR6, and some of the interior pages have PR5. That's clearly interesting to some people, because I periodically received requests for information about buying links from Chess Hounds.
I did a little searching around the web to get an idea of how much I could charge per month for links from these sites and put together a page on each (look for the "become a sponsor" link) to outline the terms of sponsorship. I wrote to a few of the people who've asked for links from Chess Hounds in the past, and today, I got my first sponsor payment (I had another sponsor from a previous deal already).
If you decide to sell sponsorships (or call it "advertising" if that sounds more appropriate for your site), be sure to watch the PageRank of the sites you're linking to. If they're using unscrupulous techniques to try to boost their PageRank and get penalized by Google (their PageRank drops to 0), remove the links from your site immediately to make sure you don't get penalized for linking to them (and make sure your terms allow you to do so--you shouldn't have to give a refund if they're exposing you to that kind of risk).
Take a look at my sponsorship programs for a few ideas for how to arrange things so that you don't have to spend a lot of time maintaining the program.
We'll have to wait and see just how much extra revenue this pulls in. Anything will be more than I was making before!
BTW, if you don't know what your PageRank is, check out MyGooglePageRank -- use it to generate links for your most important webpages and save them all in an HTML file on your own computer. Then you can track the PageRank for all of them from one page! Do the same for everyone you're linking to to easily keep track of their PageRank and make sure it doesn't go to zero.