For sheer simplicity, effectiveness, and measurable progress, it’s hard to beat the crunch. Avoid common errors like yanking or cupping your head — both of which take the work out of your core and can strain your neck.
How to: Lie on the floor with your knees to the ceiling and your feet hip-width apart. Looking up, take your hands to your head, lightly touching behind your ears. Pull your abs in, eliminating any gap between your lower back and the floor. Keeping that, raise your shoulders and upper back off the floor with an exhale, inhale as you lower, and repeat.
Variation: Turn a crunch into a lateral twist by taking your elbows across the body rather than lifting straight up. To hit your upper, middle, and lower abs all in one shot, pull your knees into your body as you crunch and lower back to the starting position with control. Pro tip: wearing sneakers adds extra weight and challenge for your lower abs.