Side Plank is a great exercise to work your core/abdominal muscles. By holding this static exercise for 15-30 seconds per set, you force the abdominal wall to engage and support your body weight while holding the plank position. I love this exercise because it can help you regain abdominal and lower back strength, help with posture, and aid in training the other muscles that assist in holding this position (arms, shoulders, hips, etc).

Correct form is a must with this exercise, so here are my tips...

Everything should be stacked: feet, knees, hips, shoulders
Raising the non-supporting arm towards the ceiling will help keep the upper body aligned
Engage (squeeze) your core and glutes for balance and stability
Make sure that your bottom hip is lifted to keep the body in a perfect line (TIP! Try doing this in front a mirror a few times to get the form down.)
Modifications: to make this easier, you can bend the bottom knee to the floor for additional support.