The next step is to block cookies from third-party and advertising sites. In Safari this is under Preferences > Security > Accept cookies. Click the radio box next to "Only from sites I visit" to protect yourself. In Firefox, go to the Privacy tab in Preferences, select Use Custom Settings For History from the Firefox Will drop-down, then deselect Accept Third-Party Cookies.
You can also use your browser's private browsing mode. Private browsing in Safari (under Safari > Private Browsing) isn't very effective since it still saves cookies. Firefox (Tools > Start Private Browsing) saves less, but still allows tracking for that session and doesn't stop tracking by IP address.
If you want to really browse under the radar, you'll need to install additional browser plugins. To start, I recommend AdBlock for Safari and AdBlock Plus for Firefox. These add-ons block most Internet ads and any embedded tracking. But they won't stop all tracking. To further bolster your defenses, install Ghostery for Firefox, Safari, or Chrome; it blocks Web bugs, tracking tags, and other techniques for over 200 tracking companies.
If you really want complete control of your browser, try NoScript for Firefox (the best browser security and privacy tool out there, though it takes some getting used to) and the Plugin Customs extension for Safari. The only problem is that they can disrupt the way sites themselves work.
As a final stopgap, use a privacy tool like MacScan to sweep your system for any lingering tracking information (as well as malicious spyware and trojans).
The risk: Local Shared Objects are small text files saved by Adobe Flash that function much like cookies and often evade other privacy controls. These are extremely common on major Websites and frequently used for tracking.
How to protect yourself: Visit the Adobe Flash online settings manager to restrict how Flash stores LSOs. In the Global Storage Settings panel on that page, deselect the Allow Third-Party Flash Content To Store Data On Your Computer (that will get rid of tracking). Setting the allowed storage space to 0 will let you manually approve any requests for a new LSO.
For additional control in Firefox, I recommend the BetterPrivacy plugin add-on. This will delete all LSOs when you exit your browser or after a specified period of time. Note that, if you do a lot of Flash gaming, you will want to allow LSOs for those sites.
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