Dan North has expanded on Arjen Poutsma's SOA 'holiday request form' analogy in his post 'A Low-Tech Approach to Understanding SOA'. Applying the paper form analogy to design the interactions and messages of your business processes is an approach that I really recommend.
What got my attention was the recommendation at the end of the post in the 'Avoid a universal domain model' section: Do not introduce an “enterprise information architecture” / “universal data dictionary” trying to force everyone to use the same domain model. Instead, introduce business concepts. This is effectively the higher-level, ubiquitous language that ties together all of the finer-grained domain models behind each service. The services use the enterprise-level business concepts when interacting, which decouples the service consumer from the service provider and allows them to evolve independently.
This is spot on with my advice for making a business process business process information model (BPIM) and avoid enforcing a "enterprise data model" or "canonical schemas" across your services. Not only does BPIM allow for loosely-coupled, evolvable services; it also allows for the services to be semantic covenants which is important for service composition.