Never carry the nailer with your finger on the trigger. A fastener easily can be driven into a person if the nailer is bumped into him while holding down the trigger.
Keep your hands and feet away from the fastener discharge area. A nail easily can hit a knot or other hard material in the wood and bend right back through the surface.
Always disconnect the tool from the power source when left unattended.
Always reconnect the air line before loading any fasteners. Connecting the air line will reset the head valve and ensure that a fastener will not be driven unless the trigger is pulled.
Use a sequential trigger unless the application absolutely calls for the rapid application of a large number of fasteners (i.e. roofing). A sequential trigger is much safer than a bump-fire trigger because the sequential trigger will not drive a nail unless the safety is pushed down before pulling the trigger.