There are a number of sites that work on a revenue sharing model, and more seem to be converting to that model every day. The principle is fairly simple: you write for them, they publish your stuff on their site and you get paid a percentage of whatever they make from the materials. Some do a straight percentage, others offer an up-front payment and then a percentage after that. I do some work for sites that share revenue, most notably Suite101. The copy generates money whenever someone clicks the ads that are on the page, and the writers get some percentage of that, depending on their position with the site.
In general I try to stay away from forums because, well, they're forums. They suck up time and make you shake your head at how insane people are. Well, I recently wandered into the Suite101 writers' forum and it wasn't terribly insane. However, I did get a little freaked out by one thing.
The site gives you daily stats as to how much you are being paid per 1,000 views of your stuff. Once I visited the forum I found out that I was making 5-10 times as much as the other writers in that forum. In fact, I think I am actually pushing the curve. Apparently there is a site average for the $/1000 number, and my numbers are 2.5 times the average.
Am I bragging? Hells yes I am. Nah, my point is that I think I know how to get clicks when you are participating in revenue sharing or have your own site that depends on ad click revenue. The writers in the forum were trying to figure out why their numbers were so low, and I can tell you why. The articles they mentioned were articles about movies, books and recipes. Articles like that don't get clicks. Why? Because no one needs to click on anything. If you're reading a recipe, are you going to click on an ad for recipes? Heck no, because you already found the recipe you were looking for and read it.
The key is really to target an ongoing action. If you're writing about a subject that people are interested in hearing about, but that they will continue to research, you have a much better chance of getting somewhere with clicks. I mainly write about online publishing, Internet marketing, etc. Those topics are extremely broad, and my article will not be the last word about them.
Someone researching PPC ads or article directories is not going to read my articles, know it all and then never want to research it again. There's a lot to learn on those subjects, and it could take weeks to sufficiently learn everything they want to know.
That makes it far, far more likely that people are going to click on the resulting ads that are matched with the content. That helps me because I get paid and it helps the advertisers because it matches them up with readers who legitimately want to know about the stuff they're advertising.
So, if you are freelancing and write for a few revenue-sharing sites, or you keep a blog or two and depend on AdSense for the revenue, try an experiment. Contribute information about an extremely broad topic and see how you do. I'm guessing that you'll do well enough to continue down that path.