How to Truly Browse in Private
The big problems are the DNS cache and Flash cookies, neither of which are covered by browsers' privacy controls. DNS, which is often described as the Internet equivalent of a phone book, translates Web site names (such as Switched.com) into IP addresses (e.g., 127.0.0.1). These IP addresses are saved locally to speed up access to those pages in the future. Private browsing modes don't clear this cache, which means that somebody could tell what sites you'd been visiting just by looking at locally stored IP addresses, even if you'd cleared your browser history.
Cleaning these things up is as easy as typing "ipconfig /flushdns" into the command prompt (or 'Run' box, whichever you prefer), and deleting the contents of "%appdata%\Macromedia\Flash Player\#SharedObjects". You can also cover your tracks with Firefox's Objection extension and CCleaner, which will take care of any traces left by all browsers.
Visit Lifehacker for a more in-depth look at how 'private browsing' modes fail. There, the geekier among you will also learn how to write scripts that automatically erase all traces of your Web-based activity. [From: Lifehacker]