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.