Side jackknifes will leave your midsection (namely, your obliques) sliced and diced, in a good way.
Lying on your right side and keeping your left leg over your right one, place your right hand in a comfortable spot and clasp your left hand behind your head.
Bring your torso and left leg toward each other as you pull with your obliques.
Squeeze for a moment and return to the starting position. Not tough enough? Strap on some ankle weights!