A visual representation of an SCA composite is a crucial part of composite documentation. Many diagrams are based on section 1.2.1 Diagram used to Represent SCA Artifacts in the SCA Assembly Model Specification. Also worth checking out is a simple, but very useful, description of SCA parts from the Tuscany project.
Since I could not find one, after about 10 minutes of looking, I decided to put together a Visio stencil for SCA composite diagrams which you are free to use. Use the basic Visio connectors to provide links between services, components, references etc. Obviously you can add more service and references to components or composites as you require.
As usual, feedback is welcome, so let me know what you think and if there are any improvements you would suggest.