Plank is among the best exercises to improve your core.
How? Facing the floor, your body must lie on your forearms and toes, keeping your feet about shoulders-width apart. Your hips must stay low to ensure a correct position: squeeze your bottom and contract your abdominals to straighten your back. Open your chest and relax you neck: it is advisable to look forward and down.
Keep this position for one minute, pause for 30 seconds and repeat at least three times.
Attention: it is opportune to avoid the common mistake of raising hips, forming an angle with your legs and trunk. If possible, use a mirror to check your posture is correct.
Tip: you can decrease the intensity of the exercise by lying on your knees instead of your toes. In this case, your body will form a straight line going from your head to your knees.
Variation: to work-out your lateral abdominals, I suggest you the lateral plank: your body must lie on one only forearm and on the sides of your feet, which must almost overlie, while the other arm is raised upwards, perpendicularly to the ground. There are almost no changes with respect to the frontal plank: abs and butt are contracted, as tightened into a bust. Keep the position for the same timing as the frontal plank.