This exercise can be killer for every angle of your core. Lie on your stomach and raise up onto your toes and forearms. Your toes should be about hip distance apart with your elbows resting on the floor directly under your shoulders. Straighten out your body so you have a straight line from the top of your head to your heels and be sure that your abdominal muscles are engaged. Hold the position for 30 seconds. Work your way up to the point where you can hold the pose for a minute or more.
To work the obliques (side abdominal muscles), perform the same move on your side. Balance on your right elbow and outer edge of your right foot, creating a straight line from your feet up to your head. Hold for 30 seconds, working up to a minute or more, before switching sides.