The plank is a very old, yet considerably neglected exercise for developing abdominal strength. Being easy to perform, simply adopt a press up position, and then rest on your forearms. Only the forearms and toes should be in contact with the ground. The back must remain straight, where the ankle joint, knee joint, hip joint and shoulders should all be in a straight line. Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.
Progressions: Med Ball Plank
Same as above but the elbows are on a soft or hard medicine ball. Soft is slightly easier than a hard med ball as if offers a little more stability. Ensure the back is straight as before (see photo below). Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.

Med Ball Plank
One Armed Plank
The same exercise as the original plank, but in a press up position with one hand behind the back (see photo below). This not only works the core muscles, but will effect the muscles of the shoulder as well. It is very important to concentrate on keeping the back straight as you start to feel a burn in the shoulder area. Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.

One Armed Plank
Progressions: One Arm Med Ball Plank
Exactly as above but with a hard or soft medicine ball under the hand. Be very careful with a hard med ball as if offers considerable instability, meaning a very hard workout for both the abdominals and the shoulder. It is also easy to fall off, so take your time in getting comfortable with the posture first (see photo below). Again ensure the back is straight and the hips do not drop, breaking the line between ankle, knee, hip and shoulder. Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.

Progressions: One Arm Med Ball Plank
The Side Plank
This version of the plank is performed on the forearm of one arm while facing sideways. (see photo below). Again, a straight line is maintained through the ankles, knees, hips, body and shoulders. Do not let the hips sag to the ground. Remember to work both sides equally to avoid a muscle imbalance. Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.

The Side Plank
Progressions: Raised Feet Side Plank
Exactly the same as the side plank but with the feet raised up onto something. In this case a medicine ball is used. As always, ensure that the straight line is maintained and the hips do not sag. Hold for 30 seconds to a minute. Take a short rest between and repeat 2-3 times.
Once comfortable with the techniques, intensify them by increasing the times held. Initially 30 seconds to a minute should be OK but when increased to 2 minutes or even as much as 5 minutes, these simple drills will really work your core muscles hard.

Progressions: Raised Feet Side Plank
Please note that there is no need to perform these exercises every day or to complete all the exercises above in sequence. These are merely suggested exercises. Feel free to try them out and find what works best for you. Doing plank drills 1 to 2 times a week will most certainly help improve your core strength. Additionally, having a strong core will help to prevent all manner of potential injuries.
