Lie faceup with your knees bent and your feet flat on the floor, or knees at 90 degrees in the air (shown at right) or resting up on a bench. Be sure that your lower back is pressed firmly into the floor. Touch your hands behind your head, exhale and curl your upper back toward your hips, lifting your shoulder blades off the floor.
Lift your feet off the floor and curl your knees toward your chest. Squeeze your abs 2-3 seconds before lowering back to the floor.
Begin by lying faceup on the floor. Bend your knees and lift your legs 90 degrees from your hips, and place your hands under your lower back. This not only creates additional lumbar support but also creates a sort of "alarm" that lets you know when your spine is shifting position and arching upward. Contract through your lower abs to slowly curl your pelvis off the floor and toward your ribcage, exhaling and holding the position for a moment at the top. Slowly lower your legs and hips back to the start position, then inhale and repeat. As you become stronger, you'll be able to extend your legs and push them up toward the ceiling (called a hip thrust) by bringing your pelvis off the floor using your lower abs.
Start in the same position as the crunch, but this time think about curling one shoulder to the opposite knee (in a diagonal direction across your body) when you crunch up, which also works the obliques. Exhale on the way up and hold at the top two seconds before lowering to the floor. Repeat on the opposite side after 10-15 reps.