For great abs, you need two things: A little knowledge and a little action.
Start with the knowledge. The first thing you should know is that crunches are a lot of pain for little gain. True, they will shape and tone the outer layer of abdominal muscles but shaping jelly still only gets you well-shaped jelly.
There are three layers of abdominal muscles, and the only way to get killer abs is to work the deepest layers: the transverse abdominus and the lateral obliques.
The transverse abdominus is the muscle you feel when you sit still and tense, as if someone is going to punch you in the gut.
The lateral obliques are the ones you feel when you tense and twist slowly to one side.
That's the knowledge. The action is even easier.
There are a number of exercises you can do to work the deepest layers of your abs. But they're boring, and doing them for 5 minutes feels like an hour. They're also not as effective as the three following activities:
Dancing: Putting on your favorite tunes and dancing for half an hour is one of the best things you can do for your abs. The key is to use your hips when you move. Hip movements use the ab muscles that run around the center of your body like a belt. They're the ones that hold you up straight and contribute to good posture. A tip for this exercise: Slow and tight movement is a lot better than throwing your back out, so unleash your inner belly dancer. Think sensual, not electrified. And yes, this works for men, too.
Laughing: Remember the last time you laughed yourself silent? You couldn't take a breath and the tears were running down your face. They don't call it a belly laugh for nothing. That ache you felt in your sides was your abdominals getting a workout as good as any Pilates session. Call the friend who always cracks you up or put on the funniest movie you know of. Another nice side effect is that laughing boosts your immune system and increases your life span, giving you more time to admire those washboard abs.
Sex: Yes, really. Without going in to the intimate details, the movements involved in even the tamest rendezvous will tighten and tone your abs more effectively than crunches. Much like dancing, it is controlled movement that will reap great results. Again, think sensual, not electrified. This is also a great multitasking exercise, especially if you and your partner have been promising each other you will get fit.
One last trick that -- unlike the others -- you can do anywhere. When you are sitting, tense your abdominals and imagine you are trying to fold your belly button in half. Squeeze slowly and release slowly. A few of these when you are watching TV or sitting at your desk and you will never have to experience the pain of another crunch again.