I have been doing 3 sets of 10 hanging leg rasies every few days for the past few weeks and that seems to be helping. Try to pull your legs up at a decent speed and control them back down to the starting position. Also helps if you can stop your body from swinging too much after you bring your legs down (I usually touch a wall or something to help ensure I start from a study hanging position). Although you could try mixing up other lower ab exercises and see what works for you. Don't forget you need a pretty low bf% for your abs to show up well.