You're straining your neck because you're trying to lift your weight with your upper body and, as a result, you're flexing and trying to pull with with your neck and probably arms and shoulders which ultimately have nothing to do with the exercise and putting more pressure on your lower back than on your abs.
The key to ANY exercise is that you want to mentally focus on the muscles you're working, so in this case, focus on your abs pulling your body up.... As far as actual technique, keep your chin up and your shoulders back; if it helps, imagine that you're trying to get your chest to touch the ceiling. Also, if you keep your hands behind your head don't lock your fingers and don't clench, keep your hands relaxed and your elbows back. Your arms, shoulders and, as a result, your neck, should be very relaxed throughout each movement.