http://ezine.daemonnews.org/200309/postfix-spamassassin.html

48 5 * * * /usr/local/bin/sa-update  --channel updates.spamassassin.org && /usr/local/etc/rc.d/sa-spamd restart

To filter before acception mails, e.g. with amavisd it is quiet simple. In my case amavisd is accepting mails on port 10024 and transfer it back on port 10025. We add the following into the master.cf:

smtp      inet  n       -       n       -       20       smtpd
        -o smtpd_proxy_filter=127.0.0.1:10024
        -o smtpd_client_connection_count_limit=10
        -o smtpd_proxy_options=speed_adjust
localhost:10025 inet    n       -       n       -       -       smtpd
        -o smtpd_authorized_xforward_hosts=127.0.0.0/8
        -o smtpd_client_restrictions
        -o smtpd_helo_restrictions
        -o smtpd_sender_restrictions
        -o smtpd_recipient_restrictions=permit_mynetworks,reject
        -o smtpd_data_restrictions
        -o mynetworks=127.0.0.0/8
        -o receive_override_options=no_unknown_recipient_checks

Make sure amavisd is running and restart postfix.