The side plank is a great exercise to form your midsection. Furthermore it strenghtens the often weak muscle called the quadratus lumborum, part of the posterior abdominal wall that plays a prominent role in averting back pain.  Start out with a basic side plank on your elbow:
1. Start on your side with your feet together and one forearm directly below your shoulder.
2. Contract your core and raise your hips until your body is in a straight line from head to feet.
3. Hold the position without letting your hips drop for the allotted time for each set, then repeat on the other side.
Once you established holding a good forms for 30 sec start adding a twist to it. 10 on each side and then finish up with 30 sec of bicycle exercise.
Have fun!