Great article on on MSDNMag by Yasser Shahoud on building your Web Services around Message Structures

SUMMARY: Web Services are all about exchanging data in the form of XML messages. If you were about to design a database schema, you probably wouldn't let your tool do it for you. You'd hand-tool it yourself to ensure maximum efficiency. In this article, the author maintains that designing a Web Service should be no different. You should know what kind of data will be returned by Web Service requests and use the structure of that data to design the most efficient message format. Here you'll learn how to make that determination and how to build your Web Service around the message structure.