a variation, to get you started.
For the plank with leg raises, start on forearms and knees. Your elbows are in line with your shoulders and your head is in line with your spine. Without jerking your hips, straighten and lift your right leg behind you. Hold for five seconds, then lower. Repeat with the left leg. Keep alternating legs, up to 10 times on each side. As a variation, repeat the entire exercise with extended arms and straight legs as the start position.
For the side plank, lie on your right side, right forearm supporting your upper body, with your right elbow in line with your right shoulder. Knees are bent and stacked with calves and feet behind you. Place your left hand on your waist to help keep your hips aligned. Contract your abs and glutes and lift your hips, hold for 30 seconds and then lower. Repeat five to 10 times, then switch sides. As a variation, repeat the exercise with legs extended; instead of holding still, pulse the hips higher up to 10 times before lowering.
Do planks two or three times a week as part of any core routine.
The American Council on Exercise has a detailed look at the plank and how to make it part of a core workout.