The Four Rules of Firearms Handling
by Jeff Cooper founder of Gunsite www.gunsite.com (Not Associated :P)

Rule 1
ALL GUNS ARE ALWAYS LOADED
The only exception to this occurs when you have a firearm in your hands and you have personally unloaded it for checking. As soon as you put it down, Rule 1 applies again.

Rule 2
NEVER LET THE MUZZLE COVER ANYTHING YOU ARE NOT PREPARED TO DESTROY
You may not wish to destroy it, but you must be clear in your mind that you are quite ready to if you let that muzzle cover the target. To allow a firearm to point at another human being is a deadly threat, and should always be treated as such.

Rule 3
KEEP YOUR FINGER OFF THE TRIGGER UNTIL YOUR SIGHTS ARE ON THE TARGET
This we call the Golden Rule because its violation is responsible for about 80 percent of the firearms disasters we read about.

Rule 4
BE SURE OF YOUR TARGET AND WHAT IS BEYOND IT
You never shoot at anything until you have positively identified it. You never fire at a shadow, or a sound, or a suspected presence. You shoot only when you know absolutely what you are shooting at and what is beyond it.

About Jeff Cooper http://en.wikipedia.org/wiki/Jeff_Cooper_(colonel)