How to design a rule engine?
I cannot believe you would implement your own considering there are so many available commercially and open source.
I recommend taking a look at InRule as a great commercial option that is reasonable priced, or NxBRE in the open source space.
If you're using .NET 3.0 or later, you can use the Rules Engine of Windows Workflow Foundation without having to acutally use Workflow.
I've done this on a project, and you can use SQL or XML as the backend, and it works great. You can use the IDE that comes with the Workflow examples and put it in your own apps. It's excellent.