Whether you know it or not, web sites collect a lot of information about your browsing habits. This information usually includes unique identifying information about you, which pages you are visiting and who you are interacting with. The information is primarily used for advertising purposes. Often times this information is also correlated widely across web sites, such as the Facebook “like” button tracker. There are many examples.
The biggest problem with this kind of intrusion to privacy may be that you never know what the data collected about you is going to be used for. It might be stored for 1 day or 1000 years and may be used for advertising or fraud (especially if a legitimate web site gets hacked, which happens frequently enough) – it is completely out of your control.
There are many ways to mitigate these attacks on your privacy, and the general rule of security versus convenience applies here as well: if you want complete privacy, then it will likely be quite intrusive to your browsing habits.
The tips below should not affect your browsing habits at all, but still offer a much higher level of privacy. They are centred around Firefox, but there are probably equivalent solutions for other browsers (please add a comment if you have suggestions).
When you visit any web site, it may request that your web browser stores a HTTP cookie for it, which is a unique identifier. The next time you visit the same site, your web browser sends the same cookie, allowing the web site to track you over time and aggregate information about you.
All major web browsers have an option to not accept cookies at all, but this may lead to some web sites not functioning well. A less intrusive option is to have your web browser frequently delete all cookies. In Firefox (v. 24) you can delete all cookies each time you close the web browser in Edit -> Preferences -> Privacy -> Accept cookies from sites -> Keep until -> I close Firefox.
Disable common trackers
Ghostery is an excellent Firefox extension that disables trackers that it has in its blacklist, while not affecting your browsing experience.
Make sure blocking is on by clicking on the Ghostery icon at the top right -> Settings icon -> Options -> check the desired “Trackers” -> Save.
Delete Flash and Silverlight Cookies
A little known feature of the Adobe Flash Player and the Silverlight plugin is that they stores cookies too, independently of your web browser. You can see and delete Flash cookies at the Adobe Flash Player settings panel.
Ghostery has a feature to delete Flash and Silverlight cookies on browser exit. Click on the Ghostery icon at the top right -> Settings icon -> Options -> Advanced -> Check “Delete Flash and Silverlight cookies on exit” -> Save.
Disable HTTP referrer
When you click a link on any web site, the web site you are going to can actually see where you came from through the HTTP referer field. For example, web sites can capture your search keywords if you find them using a search engine. Again, a Firefox extension comes to rescue: Referrer Control.
Tell sites to not track you
Modern web browsers allows you to notify web sites that you don’t want them to track you when you visit them. It is up to the sites themselves to choose if they want to honour your request, so the feature is not always useful. But for example Twitter claim they support Do Not Track, so it could help.
To turn it on in Firefox, go to Edit -> Preferences -> Privacy -> Tell sites that I do not want to be tracked.
Use several browsers
For example, you could use Opera for logging in to Gmail and Facebook, while you use Firefox for everything else. That way, Google and Facebook cannot reliably track your browsing habits outside their own sites (at least not with your name).
Also note that emails from sites often contain “special links” for you, even though they link to publicly available pages like blog posts. When you click them they know that you clicked the link since you’re the only one in the world that has this particular link. LinkedIn is one example, but most sites use this trick. You could just search for the title of the link if you don’t like to share with the site that you clicked their link (and thus what you’re interested in out of the links they sent you).
None of the above hides your IP address while you are surfing, which can be used by web sites to recognize you most of the time, depending on how often you change IP address and how many other people are sharing the ones you use. To alleviate this you could use the Tor network or a VPN service provider. This will usually have noticeable effect on web response time, however.
Please feel free to add a comment if you have other tips.