Many firearms accidents occur with a presumably "unloaded" gun.
When handing off or receiving any firearms you should always check it to ensure it is unloaded. If someone hands you a gun you are not familiar with, always ask them to show that it is truly unloaded.
Keep any gun pointed in a safe direction at all times. Whether you are at the range, at home, loading or unloading, always be aware of where your muzzle is pointed. It's not just good manners, someone's life may depend on your safe gun handling.
Guns are designed so that the trigger is a natural place to rest your finger. As convenient as the trigger is, it is not a finger rest! The most widely accepted method of trigger finger safety is to simply extend the index finger in the direction of the muzzle and hold it above the trigger guard.
Never shoot at anything until you are completely sure of its identity. Always know without doubt what lies beyond your intended target.
You are responsible for every single projectile that leaves your barrel. Make sure it travels in a safe direction.