Above is a video demonstrating some common cues we use for our general population CrossFitters as they execute push-ups. Check it out and see if it helps your athletes move more correctly! We wrote out the cues below, too.
Sagging Midline
Verbal cues
+ "Belly-button touches the floor last."
+ "Lead with your belly-button off the floor."
Tactile cues
+ Place your hand on their lumbar vertebra and ask them to "Lead here" as they come up.
+ If your athlete has been doing push-ups with a soft midline they will say this version feels harder. Say "Good!"
Sagging Neck/Shoulders
Verbal cues
+ Getting your athlete oriented vertically and having them experience the sensation of trying to be taller with good posture is a much more readily available sensation most people can quickly organize. Set that as the default position to think about before getting into a prone orientation. "Get long" or "get tall" could encorage this while doing push-ups.
+ Initially explain that they need to keep their chin and nose behind the wall on the way down. I find that specifically mentioning to push their chin and nose back prevents cervical overextenion which many would otherwise default to. "head back" or something similar would work as a briefer cue once they get the concept.
Tactile cues
+ From a kneeling or standing position, use your arm or PVC to create a line from their sternum up to their nose and have them imagine saying behind it on the way down.
If they're too weak to organize this, you may want to elevate them. More on that soon!