Via RootPrompt I found a nice article titled Fine-Tuning SpamAssassin. It covers quite a bit of detail about SpamAssassin and is well worth a read.
Over time, however, many of the spammers have figured out how to fine tune their spam and bypass the default ruleset. I find the default setup still picks up at least half the spam, maybe two thirds on a good day, but too much leaks through. If the spammers are tuning their messages, I guess the only thing to do is to tune my scoring. There are at least 8 possible ways of improving SpamAssassin’s hit rate.
1. Blacklisting known offenders
2. DNS Blocklists
3. Enable Bayesian filtering
4. Reduce the point threshold for spam
5. Increase the scores on existing rulesets
6. Upgrade SpamAssassin to the latest version
7. Install more rulesets
8. Write your own rulesets
