Clear your browser caches, saved passwords, etc.
Firefox- Click Tools>Clear Browser History, etc and also Click Firefox>Preferences>Security>and uncheck remember passwords for sites, or add exceptions.
Safari- Click Safari>Reset Safari and Click Safari>Preferences>AutoFill, and uncheck User names and passwords
Google Chrome - Click Chrome>Clear Browsing Data, etc and Click Chrome>Preferences>Personal stuff>Uncheck offer to save passwords.
Finally, go to ~/library/keychains/login.keychain and have a look in there for any saved data that you don't want and delete those entries.
*and of coarse, don't check the remember me box
They're the only 3 browsers I use, but you get the gist