The side plank is an excellent way to strengthen your External Oblique muscles as well as toning up your abdominals. There are a number of different ways in which you can create a side plank. The first one is to do a traditional press up but on the upward movement simply twist your body over placing your legs side on with the floor and resting on your elbow. You can either have both feet straight together or one on the floor. Then simply push your hips as high as possible, raising your free hand at the same time, and then slowly bring it back down. This will stretch and strengthen your External Oblique muscles which are situated to the side of your abdominals.
Alternatively, simply move straight into the side plank position, placing your body sideways to the floor. Resting on your elbow/arm place your 2 feet together stretched out as far as possible. While ensuring that you do not stoop forward, push your hips as high in the air as possible together with your free hand. Then slowly move back down to the starting position. You will feel a pull on your External Oblique muscles which will tone and strengthen over time.