For any server side Web Services development, logging the incoming requests is crucial. The following code snippet gives an example. Even though this particular example uses SOAPMessageContext from JAX-WS's SOAPHandler, the approach can be easily modified to fit into other Web Services solutions.
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
log.info("SOAP Request:\n" + new String(buffer.toByteArray()));
Note that a lot of times logging comes hand in hand with incoming requests validation. Read this blog for more details.