2.) Large selection: firearms as well as AMMO! If you sell 38Super Colt's carry the ammo for them.
3.) Quit trying to rape me on used guns. I know what you have in them and I will not pay insane prices for them. Like wise don't insult me with some ridiculous low offer when I try to sell to you. Be realistic and so will I.
4.) Offer a simple/easy way for people to utilize your FFL for internet sales of firearms. Most savvy gun bugs buy on-line. Capture that market.
5.) Have a modest gunsmith service. I'm not talking handmade firearms. I'm talking scope mounting, handgun sight removal/change, sling swivel installation, bore sighting, etc. Simple stuff that can be done in a few minutes on the counter.
6.) Have a restroom for customers.
7.) Be open on weekends. Most folks are off then. Most can't get away from work to come by during the middle of a workday.
8.) Have counter help that is friendly and honest.