Minimise your browser plugins
There are a lot of browser extensions and plug-ins supposedly protecting users against tracking:
The presence or absence of plug-ins provides a large amount of information.
NavigatorPlugins.plugins(JS) is still supported by some browsers. It returns a PluginArray object, listing the Plugin objects describing the plugins installed.
A lot plug-ins themselves will happily provide websites with a large amount of identifying information about a user, including the list of installed fonts, CPU model and speed, IP addresses, username, hostname, etc. Plus plug-ins can also have their own data and cookie stores, that they allow websites to manipulate.
Apparently it is even possible to use the NoScript plugin to gather information about what sites, or files, a user accessed while in a private browsing session and also whilst using the TOR browser.