How do I enhance spamassassins detection of spam?

Out of the box, SpamAssaassin is not especially great at detecting what is spam, and what is ham (legitimate email). In order to drastically increase the detection levels, you must first collect email and sort it into spam and ham, with the bigger the collection the better. After this is done, in ssh you can run the sa-learn command to train spamassassin. The format of this command would be:

sa-learn --showdots --(ham or spam) --mbox -u username --sync /path/to/filename

Note: if you have a Maildir setup, then do not use the --mbox flag

After this, you can monitor your spamassassin log to see if the detection rates have gone up.