Just some simple things:
1. NEVER LOAD A GUN IN THE STORE. No one cares if you are "just checking" or you "want to see if the ammo fits" or what. You likely will be invited out and told not to come back.
2. Don't point a gun at the clerk or other customers, ever, even if you KNOW it is not loaded.
3. Never pick up a gun that someone else has been looking at and laid down unless you ask him and the clerk if it OK.
4. On handing back a gun, let the clerk wipe it down; don't wipe it on your sweaty T-shirt.
5. Some shops have a "one gun at a time" policy so the clerk doesn't lose track of guns (thieves sometimes enter in a group and pass guns around so the clerk loses track and the thieves get away with one or more guns). Respect that policy and don't ask to "see those five guns so I can compare."
6. Don't ever disassemble a gun without permission. Don't disassemble a gun unless you KNOW how to put it back together again. If the clerk refuses permission, live with it. No store wants guns, especially new guns, taken apart and maybe scratched and marked up. Or springs and parts flying all over the place.
7. Have some idea what you want or are interested in. Don't have the clerk taking out every gun in the place so you can "look at" them.
8. Some gun store clerks are knowledgeable, polite and interested. Some are rude, ignorant and hate you for interrupting TV watching. Give your patronage to those who treat you right. On the opposite side, don't be arrogant or nasty. You may very well know more than the clerk, but you don't need to beat him over the head with your knowledge.
9. Once you are well known as a good customer, and are respected for your knowledge and common sense, some of these rules may change, especially if you are alone with the clerk and won't be giving others bad ideas.
10. When you reach the point mentioned in 9 above, many good things will happen. You may be offered discounts; you may ask the dealer how much it costs to accept delivery on a gun you buy elsewhere and be pleasantly surprised to find out there is no charge, even if the sign says it is $40.
Support your local dealer; he is the backbone of our hobby.
Jim