Oh - that is an interesting question. Generally, I'd just use InfoPath, and show the fields task pane in the designer - it is pretty user friendly, with the way it displays groups and so forth, and for new projects, I generally design my schema before I add any controls to the form canvas, with the knowledge the schema design may change. However, I suppose you could also use something like Visio to model the data - or you can open XSD files in Visual Studio, and take a look at the structure there (including a screenshot of that).
