Use a light touch. It is easy to get a strong grip on your pliers while you are working with jewelry. Take care that you don’t squeeze too much, which may mar the wire. Use a light touch, and you will be less likely to mark the piece that you are working on.
Sand the face of your steel hammer head before you work on your wire or metal jewelry. If it is coarse, the finish can transfer to your metal and alter the texture that you are trying to achieve. Use different grits of sandpaper until the ball of the hammer is smooth, which will give a texture that won’t change your design.