When it comes to perfecting your plank form, it’s key to engage your core (almost like sucking in). This will ensure all of the muscles you’re aiming to work are activated as you hold the position. Because you’re putting weight on your hands as well, you must remember to keep your shoulders pressed away from your ears, instead of sinking your body into the plank. Otherwise, you may wind up with a sore back or shoulders instead of the desired effect.
As for the position of your head, it may be tempting to look down at your feet while planking, but this will throw off the line of your body. Therefore, it’s best to train your eyes on the floor directly between your hands to stay strong and steady, even as holding your plank becomes more difficult.