How do you make sure that rules don’t overlap or conflict when dealing with many of them in an application?

This is the kind of thing that separates a “Business Rule Management System” from a "Business Rule Engine". It’s a complex problem and one that requires serious development work from a vendor. Blaze Advisor, for instance, includes automated tests that look for these kinds of potential problems. But it’s tricky because there are cases where the overlap is justified in the application and you have to allow it. I know of one vendor that sets up rules in a spreadsheet grid and does absolute tests against all condition combinations. That works for rules with that kind of a rigid structure, but definitely cuts down on flexibility in what you can do with the rules. All the top vendors are working on this area to try and provide maximum robustness across the widest possible range of problems.