One of the early lessons that we learned was that data mapping between xml documents was a tough thing to do manually. BPEL leverages XPath extensively - thus to do it right, you need an integrated XML data mapping and XPath manipulation tool to facilitate the mapping between service calls. See:
In the OpenStorm product this tool gen's the code for the 'Assign' tag based on drag & drop interfaces. We are finding that it significantly reduces the potential for syntax errors. It has also reduced the time to create complex orchestrations by about 40%.