It's possible to modify existing rules or their scores, oreven write your own. Matt Kettler has written "A straightforward guide to writing your own add-on rules for SpamAssassin", at http://mywebpages.comcast.net/mkettler/sa/SA-rules-howto.txt . It covers the syntax, scoring, and advanced techniques.
Chris Santerre has a collection of custom rules available at the SpamAssassin Custom Rule Emporium - http://www.merchantsoverseas.com/wwwroot/gorilla/sa_rules.htm
There's an additional SpamAssassin wiki at http://www.exit0.us . This seems more focused on rules.
Bill Stearns has a collection of spammer domain rules at http://www.stearns.org/sa-blacklist/ .