Begin in plank position with your hands underneath your shoulders, body in one straight line.
Simultaneously cross your right hand over your left as you step your left foot to the left. Then simultaneously step your left hand and right foot to the left, so you're back in plank position. Your hands move together as your feet step apart.
Repeat this for three steps to the left and three steps to the right. Be sure to keep the hips low as you move, drawing the navel toward the spine.
This is one of the best deltoid exercises around and will tone your shoulders, but your abs are getting some serious work, too.