Cardio. Cardio. Cardio.

I can't tell you how often I hear: "I've been doing a ton of crunches. I mean, I do like hundreds every day. How come my stomach isn't flat?" Well, I hate to break the news to you, but doing even thousands of crunches isn't going to get you a flat stomach.

Shocked? Most people are. Here is how it works: When you want your waist measurement to shrink, you need to get rid of the fat on your stomach. The best way to strip the fat off your mid-section -- and anywhere else for that matter -- is to do cardio.

Any kind of cardio will do. Walking, running, biking, using cardio machines like the elliptical, cross-trainer, stair master, treadmill, whatever. They all work.

The key is to make sure you are exercising in your target heart rate zone for a minimum of 20 minutes. I would recommend that a beginner do 20 minutes of cardio three times per week and work up to doing 1 hour three or four times per week.

So, what's this target heart rate zone? Well, let's not overcomplicate things right now, I'll talk specific numbers later. But the idea with your target heart rate zone is that you want to be exerting yourself enough so that you are sweating at least a little. If it is difficult to hold a conversation with your friend while running or walking on the treadmill, then bingo you know you're in the right zone.

Here is an easy way to think of it. On a scale of 1-10 -- where 1 is you're sitting on the couch watching TV and 10 is you're running for your life -- you should be between 6 and 8.

So I'd tell a beginner who wants a flatter belly to do cardio three times a week for 20 minutes each time at about a 6 pace.

After you burn the fat around your mid-section, then you'll start to see those nicely defined abs you've been working so hard at with all those thousands of crunches. ; )