I have had an FFL for 29 years and SOT status for 22. I deal in mostly high-end stuff (Gatlings, double rifles, MGs, etc.) with other dealers and don't have a storefront with display cases full of "normal" guns for sale, so my advice may not carry the weight of a guy with a typical shop.
For the last year I've been doing ever-increasing weekly business teaching CCW classes, as I have my own range and a lot of interesting guns for students to shoot. I realize that people who sign up for a CCW class and show up with the class fee in hand are by definition NOT "air stealers", but some observations I've made might still translate to the retail gun trade with walk-in traffic.
1. When dealing with someone new, give generic (rather than specific) advice, and be up front about your own prejudices.
When asked for advice about a gun for CCW, I say "Handle a lot of guns. Narrow your choices down to guns that just 'feel right' in your hands. Then look at the way you dress and the things you do every day, and select a gun and carry method that will not disrupt your life.
"Your gun should be one you have absolute confidence in as to reliability and your ability to shoot it well. I prefer revolvers because nothing feels more like an extension of my arm than a S&W revolver with the right grips. However, autoloaders have several advantages and that's why I've got a dozen different ones for you to shoot and see what feels best."
2. If someone asks a really stupid question, laugh if you must, but don't take the person's dignity. A month ago, a guy asked in class "What about shooting rubber bullets for self-defense?"
Me, chuckling: "I see a lot of the others snickering, but the fact is your suggestion has been tried by some major companies. Thirty years ago, Remington had a 12-gauge police load that fired plastic pellets that wouldn't penetrate. At five feet they'd knock a man down like a Nolan Ryan fastball, but at 50 feet did next to nothing. The problem they discovered was twofold: People tend to be quick to use non-lethal weapons, which is bad police training if that weapon is a 12 gauge shotgun that might someday be loaded with regular shot or slugs. Second, we shoot defensively to immediately incapacitate. Given that there are many cases of bad guys taking multiple torso hits from regular ammo and continuing to fight, rubber bullets are a step in the wrong direction. If you're being attacked, chances are your attacker has more in common with a wild animal than he does with you. You don't want ammo that might just make him madder."
3. Some questionable-looking people may become your most loyal customers and advocates. This has been especially true for me with black customers. A guy was in my class three weeks ago with dreadlocks, sagging baggy pants, a gold tooth, and gold chains. THIS IS JUST A FASHION STATEMENT. He would be the same person if he cut his hair, ditched the chains, and wore khakis and a polo shirt.
After I had coached several students one-on-one at the range, this guy came up to me with a fixed-sight 9mm CZ-75 clone. He confessed he couldn't hit anything with it (we were at the stage where we were shooting reactive targets such as metal swingers, tin cans, golf balls, etc.) and might there be something wrong with the gun? I fired the gun twice at a water bottle about 40 feet away and both shots hit way low, though windage was perfect. The man said something like "It's no good, huh?" I smiled.
"Throw out three golf balls." He did. Then, holding 2/3 of the front sight above the rear notch, I proceeded to drive all three golf balls far downrange with three shots. His jaw dropped. "Gun's fine--the front sight is too high. That's why you were missing--always shooting low. Here, let me explain." And I took a piece of paper and explained trajectory and what happens when the front sight is too high. He asked what to do about it.
"Some people hate the idea of sawing on a gun, and would pay a lot of money to have a professional remove the sight and refit a shorter one. That might involve refinishing the slide, as the sight looks like it's brazed on. If it were mine, though, I'd just file off half the front sight. The white dot will become half a dot, or less, but it won't be any harder to line up." He asked would I do that for him. I put the gun in the vise, covered all but the front with a rag to prevent filings from getting in the action, and had the front sight shorter in a few minutes. I tossed out another golf ball, loaded the gun, and handed it to him after a very brief lecture on sight picture, grip, and stance. His first shot sent the ball flyng to the far end of the range. You couldn't get the grin off his face with a crowbar.
The next week, SEVEN of this guy's friends showed up for class!
Finally, in threads like this one, people regularly mention that they want to patronize their local gunshop instead of "Wally World" (Wal-Mart.)
I cannot fathom why any struggling gun store owner would EVER stock ANY of the same items as Wal-Mart. Let your customers buy their 10/22s and M94s at Wal-Mart. Let them buy Winchester white box ammo. Does Wal-Mart sell concealment holsters? Surplus ammo? Premium self-defense ammo like Cor-Bon or MagSafe? Police trade-in semiautos? Milsurp guns? No? Then stock that stuff.
The guy who made the comment about wanting a gun store owner with a business background was dead on.
JR
(my local shop has about 20 brand new Glocks and only one Sig and one HK which are both USED. Oh, they have one brand new Beretta, so they are not TOTALLY lost...) 
