The best way to make them harder is to lift a leg from the ground and extend it to the side. After a while you can do it with the opposite arm too. I often finish my workout with a rotation of front, left, front, right, front, holding each of them for a minute or do several cycles with 15s or 30s holds. They can be a good abs warm-up exercise and you should still be able to do leg raises and crunches afterwards.