[blake-b]

Rules for Safe Gun Handling
Rule # 1: All firearms are always loaded. Even if they are not, treat them as if they are. (The first words usually spoken after a negligent discharge are, “I thought it was unloaded!”)
Rule # 2: Never allow the muzzle of your firearm to point toward anything you do not intend to kill or destroy. (For those who insist that this particular gun is unloaded, see Rule 1.)
Rule # 3: Keep your finger off the trigger until your sights are aligned with your target and you are ready to shoot. (The only way a gun will fire is by pulling the trigger.) I am going to add unless there is mechanical issue with the gun. There has been a documented case of a Beretta 92 (M9) going off while in the holster. Again, it was a mechanical defect. We don't know the round count or history of that gun but it has occurred once I know of.
Rule # 4: Be sure of your target, it's surroundings, and what's beyond it. Never shoot at anything that you have not positively identified.

It is all about training. Your finger does not go on the trigger until you are on target and ready to fire. Always always always have your finger along side the slide/frame when you are not ready to fire. Practice that continually and it will become habit. That way, in the event you are holding your weapon and something happens like you trip, even if you flinch your trigger finger is against the slide and cannot fire the weapon. Glock and the other companies put in drop safeties for a reason. Dropping the gun will not make it go off. In fact, if you drop your gun, let it fall. Grabbing it may cause you to engage your trigger.

I think there are some companies out there that made an external safety for the glock but I cannot remember who now. I also know Glock made a run of pistols for a foreign military that had external safeties because that is what they wanted.