Do not collapse the lower back. Instead of compromising the lower back by dipping the bum, engage the core by imagining your belly button pulling in toward the spine. This will help keep the torso flat, and in turn, the spine safe.
Don't raise your butt too high. To make the best out of this exercise your body should form a straight line. Although it will feel easier to raise your butt up, keep your back flat enough so your abs feel engaged from top to bottom.
keep your head in a natural position. do not let your head drop down, think of it as an extension of your back. Keep your eyes on the floor, letting them rest about a foot in front of the hands.
Remember that is more important to keep a good form then holding for longer time. When your form begins to suffer, it’s time to call it quits. If the back begins to bow or the shoulders start to sink in, take a break.