What's a good design of XML schema

With the help of JAXB, playing with XML becomes quite a simple thing in Java. 

But when doing my project ambienthackystat, I find it not that simple now. Because though using JAXB classes are easy, you have to had a well design XML schema to generate those facilitating classes. XML schema design is a thing that I had never think about before.

I am using XML to configure the system, to define the system's functions with trigger and action pairs. It works well in our first milestone. But when we going further, I find it somehow circumscribe our design. When I thinking of some interesting functions, I cannot help thinking how to describe it with our XML file. Then it become quite frustrating when I found it quite hard to do it. The XML file is limiting my design now... Maybe it is time to modify the XML schema, but it the system, which base on that XML, will probably need to redesign too. It comes out to be a huge task all in a sudden. 

Though I still want the schema change, this time we will do it more carefully.