Muscle Group: Abdominals
Seated twists tighten your obliques.
Take a seat at the end of a bench, feet flat on the floor and placed apart. Place a broom handle across the back of your shoulders and hold it.
Keeping your head still, and not allowing your pelvis to shift on the bench, turn your upper body and shoulders in one direction as much as possible. Hold this rotated position, now rotate your torso and shoulders back toward the opposite direction as far as you can. Try not to swing, make this a controlled movement. This keeps your oblique in shape without adding bulk, since your not using any added resistance.
The great Rob Riches demonstrates Seated Twists for us below: