FreeBSD:
[prg3@paul ~]$ cat /etc/procmailrc 
SPAMDIR="$HOME/Mail/Spam"
DEFAULT="/var/mail/$USER"

VERBOSE=on
LOGFILE="$HOME/.procmail/log"

:0
* ^X-Spam-Status: Yes
$SPAMDIR

INCLUDERC=$HOME/.procmailrc

From OpenBSD:
DROPPRIVS=yes
DEFAULT="${HOME}/Maildir/"
SPAMDIR="$DEFAULT/.spam"

VERBOSE=on
LOGFILE="$HOME/.procmail.log"
INCLUDERC=$HOME/.procmailrc

:0
* ^X-Spam-Status: Yes
$SPAMDIR

:0
* ^X-BogoFilter: Yes, tests=bogofilter
$SPAMDIR