You've probably heard it before, but you really don't have to do another crunch. (No, really -- don't. They're bad for your posture, among a host of other ills.) Instead, use these exercises to firm, flatten, and contour your waistline by training your abs, obliques, and back muscles to resist bending motions and stabilize your core, which is what they're designed to do.

Perform 12 reps of each move, moving from one to the next with as little rest as you can handle (hey, I said they weren't crunches -- not that they were easy). Take a minute break between circuits, and do three sets total.

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