I've used this extremely basic procmail configuration a million times, and it's a great start for any server configuration. It passes e-mails through spamassassin (if they're smaller than 256KB) and then filters any e-mail marked as spam to /dev/null:
LOGFILE=/var/log/procmail.log
DROPPRIVS=yes
:0fw
| /usr/bin/spamc
:0
* ^X-Spam-Status: Yes
/dev/null
Of course, you can make this much more complicated with some additional customization.
