The verdict is out on whether or not crunches are any good for you. Some research proves they have the ability to sculpt your abdominal muscles, while other research suggests that the negative impact they have on your spine far outweighs the resulting six pack. It's my personal opinion that crunches can be safe and effective—provided you execute proper form throughout—but they will not by themselves whittle your middle.
Crunches primarily target the upper rectus abdominis muscles. Which is only part of the core abdominal group. So to really feel the burn (assuming you're a pro at keeping proper crunch form), you'll need to modify. And the easiest way to do just that is to hold an object in your hands with your arms extended above your chest (not your head). Keep your eyes on the ceiling as you push that object up toward it. Use a spotter if necessary. Potential objects include, but are not limited to the following: dumbbell, medicine ball, weight plate, bar, exercise ball, and my personal favorite—the BOSU.
You can even use an exercise band, but make it one that has handles. Grab a friend and sit yourself on an exercise ball. Recline slightly with your arms extended next to your ears. Hold one end of the band as your friend stretches the other end over your shoulder and behind you, creating enough resistance to challenge your abs as you crunch and release. Again, pay attention to your form throughout every crunch. Don't speed, use the entire range of motion associated with the crunch, and always...always remember to breathe.