But short story:
Booger hook off the bang switch at all times
1 - Never point the gun at ANYTHING you do not intend to destroy.
2 - Always treat EVERY gun as if it is loaded and not on "safe" until you personally visually and physically verify that it is not loaded or chambered. Even if you WATCH me clear a gun and I hand it to you, you still need to confirm that I cleared it properly.
3 - Keep your finger off the trigger until you are on-target.
There's a lot more, but those are the three biggies.
Finger off the trigger? The gun is not going to fire itself.
Treat it like it's loaded? You won't be careless with a potentially loaded gun.
Always point in a "safe" direction? In the unlikely event that it is loaded and the trigger does get pulled, it's not going to kill anyone.
Depending on the people at the range or gun shop, some people go absolutely apeshit when someone "sweeps" them... even with a gun that they know to be empty (salesman just handed it to them).
Personally, it doesn't bother me that much in a controlled environment like a gun shop.
At a range or other shooting area where I will assume that every gun is loaded and ready to fire? You're going to be "educated"
But I personally make an effort to go overboard in avoiding "sweeping" people, even at the sales counter.