A rubber mallet can be used for a variety of different jobs. Breaking in a baseball bat is one of them. Time and patience is required but a rubber mallet can compact the fibers in a bat to make it more powerful and responsive.
Step 1: Tap Gently
Start by tapping the surface of the bat gently. Cover every angle that you can. Your bat will be brand new so you mustn't hammer too hard straight away. This will cause the surface of the bat to dent which will ultimately affect the performance.
Step 2: Pick up the Force
Oil the bat with a wood oil and leave to soak so it can treat the fibers. Let the oil dry and tap the surface of the bat again. Mix up your application of force by gradually hitting the bat harder and harder.
Step 3: Improvise and Swing
Take your mallet and bat outside in warm weather conditions; the bat can crack in the cold. Hang the mallet from a strong piece of string, wire or leather so it dangles in an open area. Swing the baseball bat and hit the face of the rubber mallet each time. This will increase the force at which the mallet touches the wood to replicate playing conditions.