Remember to keep an eye on the statistics at first and later you can set up as many websites and as many campaigns as you want. You will have access from within the advertisers publishing center to see reports that are automatically generated to track the income for you. They are on a monthly basis and you can easily spot patterns of increase or decrease in income this way.
Google will even send out an optimization report that analyzes your site and gives their publishers ideas on how to increase their income. You should pay attention to these reports and any new ways of earning income offered by your preferred advertiser. Getting in on the ground floor of a new advertising venture often leads to big bucks for being an early adopter! Try to tweak your advertising campaigns to generate steady residual income. Pay attention to which pages and topics are getting the most interest. Think about expanding these sections and adding more advertising on more pages. Add products too to get multiple streams of income.
Also pay attention to the deadbeat pages. These are pages that don't make enough income to justify their existence. It may be you want to remove them so you don't have people wasting their time and yours by landing on them. However, if there is a way to change them, learning from the other pages that are pulling in more revenue, then do so. This can be a trial and error way of learning how to tweak problem pages into more respectable income producing pages.

It will take some time to learn the type of advertising, format, placement, products and services that work best for your audience, but once you get the hang of this, it's a simple process to take what you've learned and do it for a different site too.