Nobody likes rules, but some truly are there for our own protection. The four rules of gun safety fall under this category as far as I'm concerned and must be followed at all times.
Rule #1: Treat every firearm as though it is always loaded.
Unloaded guns have killed more people than you could image. We have all read the news stories of the person who gambled that the gun was unloaded when it wasn't, don't be that guy.
Rule #2: Never point a firearm at anything you are not willing to destroy.
Keep your muzzle in a safe direction, ALWAYS. This includes when your wiping it down at the bench, showing your friend, etc. Do not point the gun at another person, intentionally or otherwise, be aware of your muzzle and who/ what you're pointing it at. Nobody likes to be swept, and it only adds potential for disaster.
Rule #3: Keep your finger off of the trigger until you are ready to shoot.
This one is simple enough, index your trigger finger on the slide, frame, or trigger guard of your firearm when you are not on target. Off the target, off the trigger. On the target, on the trigger. Almost all negligent discharges happen because the trigger was pulled prior to the sights being on target, if there even was a target present.
Rule #4: Always know your target and what's beyond it.
If you are going to shoot, don't forget that your bullet can and will continue beyond where your target is located. If you cannot be 100% sure of your target and what is beyond it, you cannot safely take the shot. This comes into play much more in the hunting world than the target shooting world, but it is none the less important.
Side note, If you have children in the home don't hide your guns from them. This only creates curiosity. Curiosity was said to kill the cat, and there have been plenty of uninformed children who have died at the end of a firearm that they just didn't understand. Don't let your kids or their friends become one of these statistics. Utilize proper safety and train your children to tell an adult when they find a firearm. See my post on the Gunvault pistol safe if you are in need of a quick access gun locker for your defensive pistol. I keep mine locked while I'm away and unlock it when I return home. It's an extra level of protection if you don't already have a method of locking your nightstand gun.
If you think your child is too young to learn about guns, think again. Do you want their first experience with a gun to be when they find yours or one of their friends parents firearms under the bed? No, you don't. While your children may be too young or irresponsible enough to shoot yet, they are never too young to learn the basics of firearm safety.
I suggest use of toy's and or BB/ airsoft guns as a training aid for children. Start them out with a toy gun, or Nerf gun. Teach them four rules of gun safety with something harmless first. When they prove, by demonstration (without you reminding them), that they are safe with the Nerf gun, then move up to a BB gun, say a Red Rider or an airsoft gun. Then finally introduce them to real firearms after they have proven trustworthy with the BB/ airsoft gun.