Focus on the solution not the problem
How often do you hear people on court muttering, or sometimes screaming to themselves? I know you have all heard coaches telling you that these emotions don't help, but you know, some people are just more vocal than others. So while keeping a calm exterior is ideal, in the meantime, let's make sure our outbursts and our mutterings are all positive. Instead of saying, "stop hitting it in the net" in frustration, try this, "come on, get it over the net". This still allows some venting but it turns it into a positive vent. If you are then mentally focusing on the solution to the problem rather than the problem itself you are much more likely to succeed. Get your head into how to fix what went wrong, rather than thinking about the error! You would be amazed at the results!