A great technique here is to adapt an approach more typically taught for the design of object-oriented systems: class-responsibility-collaboration (CRC) cards. With CRC cards, you write on one index card the name of the class, what its responsibilities are, and who it collaborates with. When working through a proposed design, for each service I list its responsibilities in terms of the capabilities it provides, with the collaborators specified in the diagram. As you work through more use cases, you start to get a sense as to whether all of this hangs together properly.
