While experimenting with Adsense you have to know if your last modification has changed anything. So far, Adsense Publishers don’t have access to more interesting statistics, I hope it will change soon. However Google tries to help us - gives us AD channels. It’s quite nice feature, but it’s still not enough…
But don’t worry! There are also few powerful and free tracking scripts!

Aren’t you Google Adsense Publisher yet?

Channels

Who can better describe what are the channels then Google?

Channels enable you to view detailed reporting about the performance of specific pages and ad units. By assigning a channel to a combination of pages or ad units, you could track the performance of a leaderboard versus a banner, or compare your motorcycle pages to your automobile pages. You can even create a channel to track each of your separate domains, so that you can see where your clicks are coming from. While channels can be used to track performance and revenue, they will not have any effect on your earnings or ad targeting.

There are two types of channels:

• Custom Channels - gives us more power - we can track each AD unit separately(for example we can track clicks and impressions of advertisement in sidebar), but we have to generate advertisement code once again with channel selected.
  We can find it in Main Adsense Page -> Adsense Setup -> Channels.
  
• URL Channels - let us to track impressions and clicks by url, we don’t have to change advertisement code.
  We can find it in Main Adsense Page -> Adsense Setup -> Channels -> URL Channels.
  

External tracking - JS + PHP

Many people make lots of money creating Adsense tracking systems, but I suppose that you aren’t interested in paying 70-120$, aren’t you?
Fortunately, there are also free alternatives:

AdLogger

It has lots of features:

• Track and log when a visitor clicks on one of your AdSense ads
• Optionally receive an email after every reported click
• Receive an email after every reported multiple click
• Secure administrative backend
• Know the total number of clicks and the total number of unique visitors that clicked
• Know the IP address, hostname, user agent, browser information, operating system of the visitor who clicked on the ad
• Determine which advertisment was clicked
• Get past click performance for a specific date or range of dates
• Find your best performing pages
• Custom channels help you monitor which pages are receiving clicks
• Block ads for a specific IP address or range of IPs
• Automatically block ads for visitors who are clicking too many within a certain time period
• Know exactly when, why, and for whom ads were successfully blocked
• If AdSense ads are blocked, you can display an alternate advertisement
• Code generator page to make installation and setup easy
• Synchronize all of the reporting to Google AdSense server time

Here’s a main administration page screenshot:

It would be great if clicks count provided by Adlogger was true… It shows about 36 click, but I’ve received only about 5.
I think that Adlogger is NOT a good tracking script, because it’s too inaccurate.

asRep

There’s only evaluation demo for free, but it has only less features and it’s not time limited!

Features in free version:

• Log reports: You can see the details of every individual records
• Overall total reports: It shows the grand total of your domains over any time intervals
• Domain reports: It shows the results for your individual domains over any time intervals
• Referring Domain reports: It shows the clicks, impressions and searches originating from a referrer domain
• Visitor IP reports: It tabulates the breakdown of the stats per visiting IP

Main report screenshot:

External tracking - JS + Google Analytics

Yes! We can also use Google Analytics account to track clicks.
I think that this method is not a very good - it’s a bit hard to implement and access to statistics is not very good.
However, if you are interested in this topic, you can read more about it here: Tracking AdSense clicks with Google Analytics.