I've worked at many music stores. I've worked at a mom and pop shop and the dreaded corporate giant (you know the one).
Here's what I've learned:
Learn what the gear does. Read the manuals, demo it in down time. If you BS an answer when a customer asks it WILL come back to haunt you. If they say, "what does this button do?" and you don't know, say "I don't know, let's look it up." Don't make up an answer. Otherwise word gets out that nobody knows anything at your store.
Keep the guitars tuned and looking nice. During downtime pick a row of guitars and polish the fingerprints off of them, tune them, and put them back on the wall.
Be humble and friendly. You'll get all skill levels in your store. From the beginner to the pro. The beginner is looking for your help so give them advise and guidance without being a jerk. The pro knows more than you. If you cop an attitude he will humiliate you and put you in your place before you know what hit you.
You will get some stupid people with stupid questions. Be patient and don't lose your cool.
Develop a relationship with your product rep. If they know you and like you they can work miracles when asking for a special order, or maybe even hook you up with some free schwag.
Above all remember you're there to work. A music store can be a fun place to work, and you get to play guitar sometimes, but only after the pricing is done, the toilet is cleaned, the strings are restocked, the drumsets are put together, the counter is cleaned, the trash is taken out, the floor is vacuumed, the cymbals are polished... you get the idea.