For deadly-accurate spam filtering, combine a well-trained bogofilter with SpamAssassin. Here's how.

Add the following lines to your procmailrc file, before you run SpamAssassin:

* ? bogofilter
    | formail -I "X-Spam-Bogofilter: yes"

Add the following lines to your /etc/spamassassin/ file:

header    BOGOFILTER  X-Spam-Bogofilter =~ /yes/
describe  BOGOFILTER  Message has too many bogons.
score     BOGOFILTER  5.0

Presto! This plugs almost all the holes in SpamAssassin's defense, and uses SpamAssassin's auto-whitelist (you've got it turned on, right?) to protect against false positives.