Check the vise for cracks or other damage before clamping a workpiece in it.
Use a vise large enough to hold the work without strain.
Place the workpiece in the vise so that the full clamping surface of the jaw supports the workpiece
Keep the workpiece in the vise close as possible to the jaws to prevent vibration when sawing, filing, etc.
Support the end of extra long work with an adjustable stand, saw horse, or box rather than putting extra strain on the vise.
Keep all threaded and moving parts clean, oiled and free of chips and dirt.
Use jaw liners in a vise where there is any possibility of marking the work.
Replace a bent handle and worn jaw inserts.