According to Jimmy Fusaro, a master teacher of kickboxing in New York City, it is a good idea to focus on your abdominals.

Without extremely strong core muscles -- the muscles of the torso, abdominals, and lower back -- it's virtually impossible to perform well in the kickboxing ring. In fact, without strong abs you will never get your leg up for the kicks.

Many of my clients leave ab work for the end of their routine, so I encourage them to occasionally put abs near the beginning when their energy level is at its highest. Any and all ab exercises are effective. I recommend a routine that includes crunches, side crunches, and reverse curls at the very minimum.

If you have access to a partner, one very effective ab exercise is to start in "crunch" position (making sure your lower back is flat on the floor), holding a weighted ball against your chest. As you crunch up, toss the ball to your partner (who is standing above you by your feet). He or she should then toss the ball back as you descend to the starting position.

There's a new ab machine cropping up at many gyms that puts you in a seated position facing a rope that is attached to a weight. Hold the rope close to your chest and then flex forward, moving your upper body toward your knees. These machines are excellent for working abs, so if your gym has one, by all means use it. If not, standard ab work done properly on the floor and supplemented by some basic strengthening for the lower back will work just fine.