In the web.config in between the <configuration></configuration> tags

<system.diagnostics>
            <sources>
                  <source name="System.ServiceModel"
                              switchValue="Information, ActivityTracing"
                              propagateActivity="true">
                        <listeners>
                              <add name="traceListener"
                                    type="System.Diagnostics.XmlWriterTraceListener"
                                    initializeData= "WCFTrace.svclog" />
                        </listeners>
                  </source>
            </sources>
            <trace autoflush="true" />
      </system.diagnostics>