It's not crunches (sorry, spoiler alert).

Those are basically a waste of time for tightening your waist.

One of my favorite exercises for really digging in and hitting those "deep" muscles of the core (the obliques and the transverse abdominis)...is the Pallof Press.

It's a great "anti-rotation" exercise done using a cable (or band) where you stand perpendicular to the pulley with your hands in together on the cable handle.

Then just extend your arms in front of you, putting greater torque on the core.

The pull from the cable forces those deep core muscles to engage strongly to prevent rotation of the torso (i.e. anti-rotation). This is one of the primary functions of the obliques and transversus...and these are the muscles you want to target to tighten your waist and flatten your stomach.

Now...I've taken that already very effective exercise a step FURTHER by adding a dumbbell...

Perform the exercise exactly the same, except this time, hold a dumbbell in your hands as you're doing the exercise.

You're not only hitting the anti-rotation, but also the anti-flexion (a.k.a. core support/bracing) function of the abs.

It's hard to see in the first picture here, but I've got a dumbbell held in my hands.

Now you can see it held out away from the body. This is essentially doubling the leverage torque on your body. I'm using a 25 lb dumbbell here, just fyi. Do 6-8 reps on this side.

Then switch to the other side and repeat for 6-8 reps.

You'll need to experiment with the weights you're using, both on the cable stack and with the dumbbell. You don't want to go so heavy on either one that it overwhelms the other...you want about equal resistance on both.

This is a great exercise to use as part of your warm-up (since it's not super-demanding on your core strength and the bracing helps squeeze blood out of core storage and out to the working muscles) or as a finisher to your abdominal training.

And it WORKS.

If you're interested in more waist-tightening/stomach-flattening exercises like this, I would HIGHLY recommend you have a look at my book the "30-Second Ab-Solution."

First, it's cheap (only 7 bucks) and it's got a TON of simple exercises, just like this one, that you can use almost ANYWHERE to flatten your stomach, tighten your waist and reduce back pain.

Check it out here..these are literally "no sweat" exercises that you can do just 30 seconds at a time throughout the day, essentially "retraining" your abdominal muscles to stay tight instead of bulging out.