If you hold up your camera in a position where you are composing and focusing, you can kinda feel all the muscles being worked. So, what you should do is go to the gym and train all the muscles that make up a hand. Shoulder, tricep, bicep, forearm, wrist.Do high reps for stamina.
The way you depress the shutter also plays a big part in camera shake. You may read or heard that the correct method is not to snap at the shutter button, but to place your finger on the button, hold your breathe and press gently.