This is what I use (in my /etc/procmailrc), but your mileage may vary:
:0
{
BOGOFILTER_DIR=/var/procmail
:0fw
| /usr/local/bin/bogofilter -f -e -p -l -c /var/procmail/bogo
}
/var/procmail/bogofilter.cf ( cut the filename short here so
it would fit on one line) is the bogofilter
config file.