We all have this problem, I think, at some point.
Dieting will help but like every other body parts, you need to make those muscles workout a bit... by twisting your body. Also, unfortunately, abs and obliques are usually not blessed with the most "exciting" workout exercises. Motivation is so important.
As Chunner mentioned, some sports like boxing and kickboxing are really good because you are forced to twist your body and they offer at the same time a very good cardio. Few sports do. Olympic wrestling is another good sport for that.
It is basically working out core muscles and doing cardio at the same time.
If you are not too much into martial arts and fighting sports, try to see if you can find classes of Body Combat. Here in Toronto, Goodlife Gyms offer those. It is a cardio class based on martial arts (karate, kick boxing, boxing, muy thai and other martial arts). May be your answer.
Good luck :-)