Always use the right hammer for the job, it will make the job a lot easier and avoid possible damage to either the hammer or material being worked on.
Never ever try driving nails with the side of the hammer head. The metal at these points is not hardened as the striking face and could be damaged.
When assembling delicate work, use a piece of scrap wood between the work piece and the head of the hammer. This will prevent damage to material being worked on.
Use a nail punch to counter sink nails into timber, again this will prevent damage to the work piece. A nail punch has a flat end (or slightly, concave) to fit the nail head rather than the pointed end which other punches have.
Where the handle is held in the hammer head by steel wedges, check regularly to ensure the wedges are tight. Timber handle can shrink in dry conditions.
If a timber handle does start become loose, place the head in water overnight, the water will cause the handle to expand and re-tighten the head.
If a hammer tends to slip off nails, roughen the face of the head using a medium abrasive sand paper.
Always wear safety glasses when driving masonry nails or breaking up concrete.