Go to "Account & Settings" on the top right, then "Privacy & Settings." Under "Privacy Controls" is this option: "Select what others see when you've viewed their profile." If you click it, the above pop-up window will appear. Select the "You will be totally anonymous option" and no one will see when you view their profile! This will limit your ability to see who visits your profile, but you can always change it back to normal when you're done being anonymous.