apt-get install spamassassin sa-compile

#! /bin/sh
sa-compile
chmod -R go-w,go+rX /var/lib/spamassassin/compiled
kill -HUP `cat /var/run/spamd.pid`
exit 0

Run from cron once per day, week or month.

#! /bin/sh
sa-update --nogpg
compile-spamd.sh
exit 0