Are you doing them fast or slow and controlled? By being sure to do them slowly, and recover back to the original position (without your neck touching the ground), with my leg outstretched and the other leg coming back up, I generally do 2-4 sets of 30. One time is left elbow touch right knee, right elbow touch left knee. Your ribs shouldn't be sore from doing this exercise. You should be feeling it in your stomach, especially the lower part around your gut and below your waistline. If you don't feel it there then you're not bringing your legs close enough to their starting position. While doing them, your body should undergo a slight twisting motion every time you sit up to touch your left elbow to your right knee or your right elbow to your left knee. If you don't find that it's at least starting to burn by the time you reach between 5-25 reps then you're not doing them properly.