Understanding the Obliques
It takes a unique understanding of the anatomy of your obliques in order to get the most out of working them. They don't quite act like other muscles do.
Most muscles have a very direct line of pull. Like a biceps muscle (to the left). The fibers of your biceps muscles only go in one direction. We'll call this North to South. Because these fibers only run North to South they lack the ability to go East and West. If your arms need to move East and West they must be enacted upon by other muscle groups.
But, your obliques are different. The muscles of your obliques are comprised of 2 different layers of muscle fibers. These fibers run on angles between East and West. The internal layers have fibers running down and backwards, toward your rear pockets. The external layers run down and forward toward your front pockets. The cool thing about this is because of how the fibers are angled, they are unlimited in terms of their range of motion. Not only can they move your trunk in linear movement patterns of North and South, they can also move your trunk East and West. With the added benefits of rotational movements kicked in for good measure.
Yep, your obliques can move your trunk in EVERY plane of motion. Once you understand this, it becomes easier to work them properly and with efficiency.
Therefore Movements which shape and firm your obliques are the ones that include all of the movement patterns your obliques are capable of. When shaping obliques look for movements that provide resistance in every plane of motion. These include the Frontal Plane (up/down from the front), Sagittal (up/down from the side) and Transverse (rotation).
To the R is a good example of a movement that includes movement in all three planes. It is intended for the abdominals, but also has the added benefits of sculpting and shaping biceps.
The boys of 0715 (Troy & Drew) getting their "swole" on.