Collapsing your lower back
According to greatist.com, instead of compromising your lower back by dipping your butt, engage your core by imagining your belly button pulling in toward your spine. This will help keep your torso flat and, in turn, your spine safe.
If you want to get super technical, have a friend gently place a broomstick or yardstick on your back. The top of the stick should make contact with your head, and the bottom of the stick should rest between your buttocks.The stick should also make contact right between your shoulder blades for proper alignment.
Reaching your butt to the sky
To really get your core working the way it should in the plank position, keep your back flat enough so your abs feel engaged from top (right below your sternum) to bottom (directly below your belt). Just don’t dip your tush too far toward the floor. While the focus may be on keeping your hips, butt, and back in the proper position, form isn’t just about your core and lower body in this move.
It’s important to think of your head and neck as an extension of your back. Keep your eyes on the floor, letting them rest about a foot in front of your hands, which will help keep your neck in a neutral position.
Forgetting to breathe
It’s human nature to hold your breath when you’re in a strenuous position. But denying yourself oxygen can bring on dizziness and nausea, which are unpleasant at best and dangerous at worst. Quality trumps the quantity of seconds ticking away. When your form begins to suffer, it’s time to call it quits. If your back starts to bow or your shoulders start to sink, take a break.