As easy as adding:
To your /etc/rc.conf.
I must warn you that spamassasin is an incredible hog.. if you get high volumes of email you may want to use a procmail/maildrop recipe to only pass certain types of emails to spamassasin (i.e. ones under 150Kb for example). If you're looking at a milter for high email volumes then I suggest using D-SPAM instead..