smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
Also, if this is the first SASL service installed on the machine, make sure there's an appropriate SASL plugin installed:
That should do it! Postfix will log into the relay host using the smtp auth username and password. It's possible to set different logins for different servers, by adding more lines to the map file. Security options must be cleared to allow plaintext logins.