Get your athletes to regularly practice developing an awareness of when they start to drift from a proper focus, and getting themselves back. Throughout practice routinely call their awareness to this mental skill. Help your athletes understand that the main difference between their best and worst performances has to do with their preperformance self-talk and thoughts. That what they think goes into their bodies and can tighten their muscles and rot them of their coordination, reflexes and speed. The athlete is not thinking and is on automatic utilizing effortless effort.
Poor performance is about doubting, over-thinking, analyzing, evaluating and trying too hard.
You accomplish this by giving them one or two specific things to focus on for the event. By narrowing concentration, the athlete has more of a chance to slip into an automatic meet mentality. Athlete's self-talk about the meet, competition, events, crowd, etc. Negative past thoughts will bring you down and uncontrollable future thoughts of the outcome will do the same. Listen only to those things that do the same for you.