Your CPU automatically reduces its clockspeed when it isn't under load (ie, when you're browsing the internet) specifically to save power. There isn't much you can do in this regard.
If you want to really save some power, you can undervolt your CPU. Here is a nice guide to show you how to do it:
http://www.notebookforums.com/showthread.php?t=70943
The basic idea is that your CPU is getting more voltage than what it needs to operate. Using a utility like RMClock, you reduce the voltage going to your CPU without reducing clock speed. If done correctly, there is absolutely no drawback... no speed loss, no stability loss, just better battery life and a cooler laptop. By undervolting my laptop, I increased the battery life from about 2:30 to 3:15.