Extend your upward reach. The farther your arms are away from your trunk, the harder it makes a crunch. For example, try holding a 5-pound bag of flour in one hand while your elbow is bent. Compare that with holding it with your arm extended. You'd think it got heavier. Your abs will feel the same way when your arms are extended over your head.
Get a leg up. Make that two. Extending the legs straight up in the air knocks out any assistance from the thigh muscles, and you have to work harder to stabilize the pelvis.
Fight gravity. Performing abdominal exercises on an incline (head pointing down for crunches) is harder work because you're working against gravity. There's just one catch--don't hook your feet on the holders if you're using a traditional incline board. That allows lower body muscles to help with the work.
tips: how many crunches?