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.
Timney's 10 Commandments of Gun Safety
From the back of a Timney trigger package - some of them are probably overkill
Do not rely on your gun's safety. Treat every gun as if it were loaded and ready to fire.
Never cross a fence, climb a tree, or jump a ditch with a loaded gun.
Never load or carry a loaded gun until you are ready to use it.
Watch your muzzle so your companions do not have to.
Store guns and ammunition seperately and in locked storage.
Do not shoot unless you are absolutely sure of your target and what is beyond it.
Know the range of your gun. Remember, even a .22 rimfire can travel over a mile.
Always wear ear and eye protection when shooting.
Always be sure the barrel is clear of obstructions. Carry only the proper size ammunition for the gun you are carrying.
Alcohol and gun powder do not mix.
Safety Always
Got kids? - Better get a gun safe! Gun safes are available through a variety of vendors.
-- AdamSmith - 26 Feb 2006