well this is similar to what is proposed in the yerkes-dodson effect. Over stimulation during a sport can decrease performance. My tip would be to errm read up on this psychological concept and try to come up with a solution yourself. I could easily just say something like 'calm yourself down before an event by taking 2 minutes to effectively clear your mind' but i think by actively looking for a solution your performance will greatly increase because you will understand what is going on, and how to focus your mind on the task.
Its like in poker, if you dont think enough then you lose because you dont consider the possibilities what your opponents have and if you think too much you start to second guess your plays and thus dont perform too well.