Start a "niche" web directory and add about 500 listings to it. Throw Adsense on it and see how you do. You might also want to try out affiliate marketing.
Have you tried Sponsored posts? Advertisers will pay you to write reviews of their product/site and post them onto your blog. Depending on your PR you can earn anything from $5 to $200 per article. You will find a list of them on my blog (link in sig). Just click on the Make Money link in the menu and look for 'Sponsored Posts' for a list of companies that offer this service.
Did the half of the people who replied to this thread earns a $100 per month, or they've just want to help? Thanks to those informative replies, but I've learn just a little! Haha.. I still really want to make $100 per month, and it is really very hard! And by the way, please do not reply a promotional, thanks..
In my opinion , SEO work is the most important thing. And reorganize your site a little . It's too messy.