In addition to these, bear in mind that if you're too tired or irritated to follow these rules, you probably shouldn't be handling a gun at that moment.
The Absolutes
Never point the muzzle at anything you don't want to shoot.
Treat every gun as if it is loaded, even if you know it isn't.
Keep your finger off the trigger until you're aiming at your target.
Bullets cannot be recalled once fired: you are responsible for the safety of everything between the muzzle of your gun and the place where the bullet comes to rest.
Additional precautions some folks believe in
Leave semi-auto chambers empty - it's almost impossible to rack a slide accidentally, but easy to disengage a safety.
Don't store your guns loaded unless you have to for self defense.