InfoQ have just published a very good article , explaining both why you want to use a Rules Engine and how to do it. In Summary:
‘In general, you might consider a business rule solution if you need to externalize business rules, support rapid change and empower business users to change business rules…
Rules are simpler, easier to write and understand, and use facts that are treated in a less interconnected manner’
If you are in interested in learning more, here are links to two articles I wrote for O’Reilly on JBoss Rules / Drools:
You might also be interested in checking out JBoss Seam which makes it very easy to plug the JBoss Rules Engine into the presentation (JSF-Web) and Database (Hibernate) layers.