For a quick and efficient way to target the muscles that line your waist (and work the rest of your body, too), try this move from Nichols:
How to do it: Come to a side plank position with the legs staggered (top leg forward, bottom leg back), hand directly under your shoulder and top arm extended (A). Inhale and reach your top arm up and over (next to your ear) while lifting your bottom hip even higher from the floor, flexing the spine while keeping the hips and shoulders square (B). Exhale and lower your bottom hip halfway down toward the mat, sweeping your top arm down next to your thigh (C). Continue back to the lifted (A) and lowered (C) position without pausing to complete 15 reps before switching sides. Work up to three sets of 15 on each side.