Don’t use Windows. Just don’t. This also means don’t use the Tor Browser Bundle on Windows. Vulnerabilities in the software in TBB figure prominently in both the NSA slides and FBI’s recent takedown of Freedom Hosting.
If you can’t construct your own workstation capable of running Linux and carefully configured to run the latest available versions of Tor, a proxy such as Privoxy, and a web browser, with all outgoing clearnet access firewalled, consider using Tails or Whonix instead, where most of this work is done for you. It’s absolutely critical that outgoing access be firewalled so that third party applications cannot accidentally leak data about your location.
If you are using persistent storage of any kind, ensure that it is encrypted. Current versions of LUKS are reasonably safe, and major Linux distributions will offer to set it up for you during their installation. TrueCrypt might be safe, though it’s not nearly as well integrated into the OS. BitLocker might be safe as well, though you still shouldn’t be running Windows. Even if you are in a country where rubber hosing is legal, such as the UK, encrypting your data protects you from a variety of other threats.
Remember that your computer must be kept up to date. Whether you use Tails or build your own workstation from scratch or with Whonix, update frequently to ensure you are protected from the latest security vulnerabilities. Ideally you should update each time you begin a session, or at least daily. Tails will notify you at startup if an update is available.
Viciously drop cookies and local data that sites send you. Neither TBB nor Tails do this well enough for my tastes; consider using an addon such as Self-Destructing Cookies to keep your cookies to a minimum. Of zero.
Your workstation must be a laptop; it must be portable enough to be carried with you and quickly disposed of or destroyed.
Don’t use Google to search the Internet. A good alternative is Startpage; this is the default search engine for TBB, Tails and Whonix. Plus it won’t call you malicious or ask you to fill out CAPTCHAs.