Bodybuilders will generally shy away from doing a lot of oblique exercises such as seated twists or side bends with any kind of weight.  Developing the obliques will add mass to the mid section and take away from the V shape bodybuilders need to create definition between their waist and their shoulders.  Big obliques will make them look smaller.

Boxers require functional obliques in order to twist the torso and to pull elbows down to block body shots.  Tightening the obliques can be done with light weights and high reps and will not add significant amounts of muscle.  If you care about the size of your midsection, then keep the weights light.

Starting Position

Stand with feet about shoulder width apart.  Put a staff or broomstick handle across your shoulders and grip both ends with an overhand grip as if you were about to squat the bar.  Bend over at the waist so you are forming a 90 degree angle between your upper and lower torso.

Performing the Exercise

Keeping your head stationary and keeping your pelvis from twisting or moving, turn your upper torso and shoulders as far as you can in one direction.  You are twisting around your waist.  Hold at the peak of the contraction in your obliques and then rotate in the opposite direction, again to the limit.  Keep the whole motion under control and prevent excess movement from lower body or swinging.

Tips

You can vary this a bit and force your obliques to react to a situation where you need to block a body shot by explosively twisting from left to right and right to left.  You will have a split second in the ring to make the twist to get your elbow in the way, so you may as well train that way now.  Be careful to remain in control though, it is easy to pull an oblique by twisting too hard and not being able to stop the twisting motion without too much strain.