Although I think the side bridge is a great abdominal exercise (see the best ab exercise of all), this is an advanced exercise and should only be attempted after the following exercises are mastered.
I teach an easy form of Pilates exercises, to which you concentrate on abdominal hollowing without flattening your back, and then alternatively lift your legs. It is possible to then have your arms move in a scissor motion over your head. In this article from Journal of Bodywork and Movement Therapies, there is a slight variation to the exercise to which you push your hands against a wall to produce an isometric contraction (not moving), within the abdominal cavity. Here is the procedure as from the article:
Lie on your back with your head about one foot from the wall. If necessary put a small pillow or folded towel under your head for slight support. Place your palms against the wall while being sure your elbows are slightly bent, lift one leg up at a time until both feet are in the air with your knees bent.
Next, push away from the wall with your hands (don’t actually move your body as this will be what is called an “isometric” effort”, you should instantly feel your abdominals tighten in the front and sides along with your back muscles so that you have 360 ° of support or tension. Notice how when you do this your lower ribs above your belly move down towards your waist. This is an isometric version of the famous “crunch” exercise. While pushing away from the wall and feeling your ribs move downward be sure to maintain a normal in and out breathing pattern. Do not hold your breath in or exhale vigorously. Just continue to breath normally. Finally, while continuing to push away from the wall, feeling your ribs move down, and maintaining a normal breathing pattern begin lightly tapping the floor with one heel at a time.