When pounding meat, there are two important factors for success. The first is plastic wrap. Don't scrimp on the amount that you use as the meat will need enough room to spread out as you pound it. The second is to choose a heavy meat mallet with a flat side.
Begin with the cut of meat in between the sheets of plastic wrap. For a chicken breast, the plastic wrap should be at least 12 inches square. Begin by pounding gently at the thickest portion of the meat. As the meat flattens out gently pound by moving slowly towards the outside of the meat so it will flatten to equal thickness.