Strengthen your abs if you want to take body shots, strengthen your chest (Pectoral Muscles) if you want to take shots in the chest. Don't worry about isolating the muscle, you just need what works for ya.

Once you have the extra muscle, you can further toughen your muscles up through Iron Body conditioning, have someone hit your abs with a medicen ball, or have them hit you all over, and increase the the intensity over time.

-Rocky