Safety first! As others have said, check every gun brought in, open every gun before handing it to customers, open it after they hand it back.(If it isn't. It should be.) If someone behind the counter does not do these things, I lose respect for them right there; they then have to earn it back.
Assume the customer knows more about the subject than you until he proves otherwise, then be tactful while educating him. If he knows more than you, cajole him into educating you.
Get to know your repeat customers; establish a rapport with them. It benefits both of you. My LGS had a bunch of 209 primers, that came in with a gun, waiting for me at a price I couldn't resist the other day- even though the guy I usually deal with does not shoot Trap or reload. When they get new .45 ACP guns in, I get to see them before they go in the case. They'll set a gun or reloading stuff aside for me without having to put money down right away, because they know I'll be back in with $$ as soon as I can.
Learn enough about aspects of the shooting world that you know little about, at least so you can answer some basic questions.At a shop I worked in, the owner did not like AR's (still doesn't, but at least he has finally put some in the racks) milsurps, and military type guns in general. (except 1911's) He is into the Browning O/U's and Benelli autos, and sells lots of them. (nothing wrong with that, but some balance is nice.) Counter guys are retired cops or kids into duck and pheasant hunting mostly. When I worked there I tried to impart some knowledge about military type guns, and some of it stuck, but they are missing out on a lot of sales.
Don't expect to make a lot of money, as mentioned above, plus be careful with the store's associate purchase policies; Texans have a saying, 'gun poor', and working at a gunshop is one way to get there.