Flip the plank and light up your backside.
Drive your hips high and don't let them sag. Keep your chest open and shoulders down.
Flipping the plank face-up reverses the whole bracing problem, gravity now drags your hips toward the floor and your core has to hold the straight shoulder-to-heel line from the back side of your body. Squeezing the glutes to keep the hips tall is what keeps the line honest. It earns its place because almost all core training happens face down or seated, and this hold trains the posture and posterior control that side of things misses.
Fatigue plus forgetting the glutes. Actively squeeze them and push your hips toward the ceiling for the whole hold, when the hips drop below the line, end the set rather than holding a sagging position.
Plant them behind your hips with fingers pointing roughly toward your feet and press the floor away, keeping shoulders pulled down away from your ears. If wrists complain, try the forearm version instead.
Start with 15 to 20 second holds with a perfect line, and build toward 45 to 60 seconds. Length only counts while the hips stay up.
