We often hear about someone making such and such amount of money from one particular form of advertising, or from one particular web site. This is all great. However, assuming that publisher does not have another income stream (whether it be a different website or a different method of income), that publisher does run the risk of having that single site / single monetization mechanism taken away from him/her. What if someone outranks you in search engines and you lose half of your traffic? What if the program you have been relying on all of a sudden decide to cancel your account? Or simply disappeared? Do you have a way to react to these types of events?
There are two dimensions to diversification. The first one is the type of income. In the English-speaking world, there are many options. There is contextual advertising like Google Adsense / YPN, there is display advertising, there are affiliate programs, there are even other types of less-traditional programs like ReviewMe, PayPerPost, TextLinkAds, and job boards. To get a flavor of the various types of monetization opportunities, visit a couple of “make money online” blogs, and you’ll see what’s out there.
The second dimension is the site. Instead of relying primarily on a single site for all, or most, of your income, you should branch out and have multiple sites. This way, if something bad happens to one of your sites (SERP drop, web host going down, etc), it’s not catastrophic to your income.