//Initialize the service
ITracingService tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));
Once initialized, you just need to add the logical and meaningful details as per your logic using below code,
tracingService.Trace(yourMessage);
Shown below is the example of message shown to the user when plug-in fails and throws exception.
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Hope this helps you how to use Tracing in CRM 2011 plugins!