Genetics play a big part in having great obliques.  With the right nutrition though, you can accomplish a lot.  Getting your body fat below 10% will make a big difference, so really watch what you eat.  Salt intake is critical, as your body will release water weight and you’ll see your obliques a lot better.  The right supplements such as vitamin C and vitamin B-6 can also help in your quest for definition.

I like to superset my obliques with another body part.  To prevent thickening in my waist, I use my own body weight and focus on pausing and squeezing the obliques.  I like to do hanging leg raises—grasp a pull-up bar, tighten your core and bring your legs up to 90 degrees.  Then, lift your knees to one side of your body, squeeze and extend them back out.  I go on one side at a time until failure, and then switch. 

Oblique knee-ups off the side of a bench also work well.  I grasp the bench behind my glutes.  I bring my knees to one side of my torso at a time, then extend my legs in front, pausing slightly before the next rep.

The only exercise in which I use light resistance is the kneeling cable crunch.  I pull the weight down, one side at a time for about 15 reps.  The trick to this move is using a full up and down range of motion.  I go two counts up and inhale at the top, then two counts down and exhale, squeezing my obliques.

Another good time to squeeze your obliques is when doing cardio.  While on the bike (lifting my glutes off the seat a couple of inches), elliptical or stair-climber, I squeeze 30 seconds for each side, and then relax for one minute.  Do this for 30 minutes, and you’ll be amazed at how sore you feel!  It’s important to stretch your obliques too.  I lie back on an exercise ball for a few minutes and that’s my daily meditation.