As players get older, the mental side of the game gets more important as well. Jack Nicholas had the famous quote, "90% of golf is mental." I think most people at the higher levels would agree that the percentage applies to baseball as well.
"Dude, you are killing us. You're front side
is just firing open on every swing."
There are certain players that just seem to kill your team. For whatever reason, they have your pitchers' numbers and nobody can get them out. When I played shortstop, here's what I sometimes would do. In between plays and especially during pitching changes, I would get into conversations with the runners at second base. If the runner was the guy who was killing us, I would try to, shall we say, "plant a virus" in his brain. I would say something like "Wow, you are on fire! You're really firing those hips" or "Man, you are really loading up and jumping at that fastball." My goal was not to compliment the hitter. My goal was to get him thinking about why he was successful. On top of that, usually what I said was purposely not what was making him hit the ball well.
Most of the time, when hitters are lighting it up it is because they are not thinking much of anything. They are just in the moment, seeing the ball well, and just letting the swing flow. Usually when hitters stop to analyze the mechanics of their swing too much, it all comes down like a house of cards. That's where the virus comes into play. Hopefully, his mind would start bouncing my comments around before his next at-bat and knock him out of his "non-thinking" pattern or at least get him thinking of something different. In the specific comments mentioned above, I would hope that he would start to over-emphasize the mechanics that I brought up. Firing the hips too early next time might lead him to pull his front side out early or maybe getting him to jump or lunge at the pitch too much would make him vulnerable to off-speed pitches.
Ethics aside, you might be surprised how much this actually works.