Architecture and DesignMake sure that all input supplied by the user to the password recovery mechanism is thoroughly filtered and validated.
Architecture and DesignDo not use standard weak security questions and use several security questions.
Architecture and DesignMake sure that there is throttling on the number of incorrect answers to a security question. Disable the password recovery functionality after a certain (small) number of incorrect guesses.
Architecture and DesignRequire that the user properly answers the security question prior to resetting their password and sending the new password to the e-mail address of record.
Architecture and DesignNever allow the user to control what e-mail address the new password will be sent to in the password recovery mechanism.