Zagz hit it right on the head. LFS' like most businesses make their money through turnover. That is selling their product. Most would go out of business if customers followed a fishless or very structured fish cycle. Most would also go out of business if they didn't sell all the water "conditioners" and pH "adjusters" and medications that are dangerous to use except in specific cases. Without these money-makers, a LFS is in need of a different angle that will be tough to manage.
Here is what I would want to see if I was envisioning my own store (in order of importance):
1.
Pristine conditions in the tanks. Quality OVER quantity. I don't want to see every species ever bred/captured, I want to see a good selection of healthy fish that are not overstocked. I want the species in the tanks to be compatible. No fancy guppies and tiger barbs, no cichlids and goldfish, you get the idea. And I want the tanks to be well-maintained (no dead fish, no poo on the substrate/bottom, and would prefer for there to be SOME decoration/hiding spots in the tank (these can easily be removed before selecting fish). I love PetSmart's cards on the tank that tell about the species in the tank. I just wish they were accurate. I would group the fish by tank size. Something like 10 gallon, 20 gallon, 55 gallon, 100 gallon. Make it obvious when entering the next area. Don't put the bala sharks next to the guppy's, don't put the ghost knife next to the tetras. Buyers will ALWAYS gravitate towards the fancy fish regardless of whether they can maintain the fish as it gets larger. They plan on *upgrading* down the road of course.
2.
Know your stuff or be honest There are so many times I hear people talking out of their arses either outright lying or making stuff up to sound smart. Just admit you have limits. Recommend a forum like this to your customers for specific questions you cannot answer, and many more people will respect you. Keep an open mind, take everything with a grain of salt, and constantly try to improve your education of the species you house, interactions, disease, etc.
3.
Teach proper procedures If I had my own store I would have stacks of printouts with simple step by step instructions for doing common tasks in the hobby. I bet 90% of the fishkeepers improperly do PWC's (don't match temp, add too much/too little/no dechlor, change too little, use distilled water etc). Same goes for feeding. You can tell them anything in the store, but once they get home, have to cook dinner, or take their kid to practice, or have a bad day, they will forget (or worse *think* they remembered what to do). If they have a piece of paper with the simple how-to's they will be much better off.
4.
Sell seed material This one comes down to pure greed IMO. LFS' make money from selling stuff, if the fish die the first couple times, they sell more fish. Profit! There is a HUGE market for seed material, just look at how many people post in the getting started forum that the LFS won't give them gravel, filter pieces, etc. If I had my own shop I would offer (at a markup of course) preconditioned seed material for a new person to the hobby. Imagine walking into a store and being told you need to wait a couple weeks with an empty tank for things that eat fish waste before you can get a tank. They will never come back to your store. Now imagine you tell them most of their fish will die if they just put them in a new tank, but surprise, I happen to have a fully seeded filter of your exact spec in the back that I can sell you. Have the house filtration system run through a sump setup with a hodgepodge of filter media from most/all of the filters you sell. I'd select one or two well established and recognized filters to make things easier (say AquaClear, Hagen, and Rena).
5.
Highly reccomend liquid test kits Demonstrate how to use them for the main tests. Show them how easy and how little time/effort it takes. Some will still refuse it, but some people will like the fact that YOU gave them the power to monitor their own tank levels. They don't need to run into the store with a cup of water and have you test it to tell them there is/isn't a problem, they can do it themselves. This empowers your customers to be more involved and wise.
6.
Refuse a sale At least in the US its your right to do so. Impress on them that you care about the fish too much to sell one when you know its not the right thing to do. Sure they might get pissed, and storm out, but I can tell you that if anyone was in earshot, they will/should respect you for it. I know I would.
Just my thoughts...