Lie on your side with your knees bent, elbow beneath your shoulder. Rest your top arm on your side, or place your hand on your hip. Brace your abdominals and lift your hips up so that your body forms a straight line from head to knee.
Hold for at least 10 seconds, building up to 30-60 seconds. You should be able to breathe normally.
Progression: Repeat, but resting on your feet and elbow, forming a straight diagonal line from your head to your toes.
Modified Curl Up
Lie on your back, one knee bent, the other out straight, hand under your lower back. Lift your head and shoulders off the floor, tightening your stomach muscles then roll the head and shoulders back down.
Repeat 10-15 times.