Building Intelligent, Rule-Based Applications?

Donald Hughes asks: "What are good approaches for building intelligent, rule-based applications? In particular, I typically build ASP.Net applications. I came across P#, which translates Prolog into C#. I also came across NxBRE, which touts itself as the first open-source rules engine for the .Net platform. Currently, the intelligence in my applications consist of data stored in the database, which is either processed in T-SQL or in a C# class using linear conditional logic. This approach works fine to a point, after which the complexity of the if/then statements becomes extremely difficult to manage. This is especially problematic when returning to a project after several months of not looking at it, or when someone needs to be introduced to it for the first time. Does anyone have some helpful advice?"