With a weight in your hands in a glute-ham bench sit-up position, perform a series of sit-ups. As a starting point, the weight can be held on the chest, but its position can be changed to adjust difficulty.
 
After completing the sit-ups, stop and hold the trunk at around a 45-degree angle, and move the weight from side to side while keeping the trunk stable and facing forward. The shoulders will rotate somewhat unavoidably—focus on keeping the sternum facing forward to minimize rotation of the trunk as a whole.

The straighter the arms, the heavier the weight, and the faster the motion, the more difficult it will be. 
 
Repeat multiple series of each exercise for each set; for example 5 sit-ups + 10 twists repeated 3 times.
 
Purpose
This is a complex that addresses nearly every aspect of abdominal training—trunk and hip flexion, isometric trunk stabilization, and stabilization against trunk rotation.
 
Programming
Perform at the end of a training session. Determine reps of each movement based on the desired emphasis and conditioning.