Some of the things I see in the weight room amaze me. Especially this one - when bench pressing, the athlete's fingers will actually begin to open up as they press the bar. What? Yeah, I don't get it either, but it happens. Little do they realize that by actually squeezing the bar harder it will help them lift more.
Make sure the athlete has a good grip on the bar. By that, I mean that the bar should be set in the palm of the hand, closer to the wrist, not up by the knuckles or fingers. If the bar is up by the knuckles, then it bends the wrist back. Also, try instructing the athlete to use a closed grip, not a thumb-less grip. Some of us more advanced lifters can get away with using a thumb-less grip. Heck sometimes I switch back and forth. But for my athletes, I make it a point to have them keep their thumbs around the bar. This forces them to squeeze the bar. As the weight gets heavier and you can see that they are starting to struggle, instruct them to squeeze the bar. Hearing that cue mid-lift will help them to realize that they might be loosening their grip.
So, SQUEEZE THE BAR. Try this cue and let me know how it works for you or your athletes.