Before beginning any movement in the quadruped or plank position, look first at the initial conditions. Is the client in the position you want them to be in?
One of the first things I generally look at is the scapula. Does it lay flush against the thorax or is it winging? Visually assess this or use your hands to palpate the scapulae. You shouldn't be able to see or feel the medial border of the scapula when the patient is in quadruped. If you are able to, it means your client isn't properly engaging his serratus anterior. In this case, cue him to "push into the ground."
Next, I check the elbow position. I love keeping the elbows slightly flexed. With the elbows locked into extension, you don't have to rely on our muscles to actively stabilize the upper quadrant. If you don't do this, TRY IT. You'll be forced to engage your shoulder girdle muscles so much more. It’ll change the way you do quadruped position! Slight elbow flexion is a progression of normal quadruped/plank position, so there is no need to do this on a client who can't even keep their scaps flush.
Remember to always keep an eye out for any compensations once you begin the exercise! TAG A FRIEND who thinks quadruped exercises are too easy and make sure they're doing them properly!! Shoutout to my CI Dr. Brandan King who taught me this cue!