I'm going to give it to you fast and simple. If you use AdSense, this is definitely an article for you.

With a little help from Google, we can make more with our project wonderful ads. When you make or edit an AdSense ad you have the option to display public service ads or an ad from another URL.

Make a simple HTML page, paste the code to your project wonderful, then save it on your server somewhere. Copy the URL of that file on your server, paste it in the "show ads from another url" option.

Now you know how to do it...
Read more if you want to know how it works...

Two basic principals
1. Advertisers want more page views
2. You have extra page views

How do I know this works? Because I've tested it myself. Plus, I subscribe to Chexed.com, so I'm obviously awesome.

Step by step instructions: (*Thanks for the suggestion Masso!)
1. Create a new add
2. Select "Adsene for Content"
3. Select "Ad Unit" or "Link Unit"
4. At the bottom of the following page page find "More Options" at the bottom
5. Under "Alternate ads or colors" select the box "Show non-Google ads from another URL "
6. Continue as usual

You can also edit an existing ad instead of creating a new one. The "Alternate ads or clors" area as long as you choose to edit an "Ad Unit" or "Link Unit."