Want rock-hard abs? Don't bother with crunches. Instead, here's a better suggestion:

According to this fit tip from Men's Health, trade in your traditional crunches and sit-ups for planks and Swiss-ball crunches. "Both are better than the standard crunch for bringing out your abs," says Tom Seabourne, a fitness expert.

I'm guessing you already know how to do the plank -- the next step is just making sure you hold it long enough to challenge yourself. Need even more of a challenge? Try bringing one knee into the chest and out again a few times, and then repeat with the other knee.

Click here for a Swiss-ball how-to video.