I have occasionally touched on what others have done and written about in this respect, and before we end I'd like to touch on two more sources.
The first is Streamlined Object Modeling [Nicola et al. SOM], which has a thorough discussion both about different rules and strategies for design and implementation of the same, all in object-oriented context.
The second is Enterprise Patterns and MDA [Arlow/Neustadt Archetype Patterns], which discusses archetype patterns for how to organize rules. I think you will find that many of the ideas there are useful for moving on from here.