Be sure when raising your feet and arm that you raise them parallel to the floor. Keep your arm and leg straight and avoid bending at the knee or elbow.
It's imperative to the exercise that you hold the position at the top of the repetition.
You must breathe to bring oxygen to your muscles. Breathing helps stabilize the core and maximize the results of the exercise.
Your body should be balanced while lying down so that each side of you is doing equal work. Be sure you are not off-center as this will create a strain on your back, rather than promote core strength.
Avoid pointing the foot when you raise it.By keeping the foot perpendicular to the floor, you keep the work in your core and get a better muscle contraction in the focus area.