I worked at my LDS before I moved. I would tell customers about better deals, that certain items were a waste, or about how to maximize their budget.....especially if they were loyal. Before I worked there, I found a great deal online and took it in to them. I asked them to match it. They did. They gained my loyalty. I'm normally willing to pay a little over "lowest" to support the brick&mortar shop nearest me, but refuse to pay 200% of what I can find it for online. Even while I had a good employee discount, I'd go online when they couldn't match it. One owner confronted me, and I told him I'd be MORE than willing to buy from him if he'd match. He didn't. I didn't. I still went back for TONS of other stuff.
One thing to note is something one of the owners of that shop told me: Dive shops aren't necessarily competing against eachother. They're competing against other outdoor activities. If your dollars are being spent on trips with another shop, you'll want gear that you might buy from MY shop. It's much better for me for you to spend all of your disposable income on scuba, and split it between my shop and others than it is for you to split your disposable income between scuba and other hobbies, and then split THAT between shops. If you're diving, you're doing the diving business good as a whole.
If a shop felt like they owned me, I wouldn't go back very much. If a shop has a good back-scratcher, I'll scratch their back to access it. As an employee, I may not have been the best upseller....but I had the best back-scratcher. I was also the highest selling employee at the shop by miles.
As far as etiquette goes, there's not too much to learn. Most of the dive industry is very laid-back (something you'll learn quickly) so a laid-back attitude is something you need to have as well (something that might take a while to get). Diving is about stress-release. It's a hobby. If a shop bends me out of shape for loyalty, grating personality (gun store owner had a full-blown yelling match with an employee because he was showing me a gun I was about to buy instead of running inventory....I haven't been back sicne), or simple "etiquette" issues then I don't enjoy it as much. If I don't enjoy an aspect of my hobbies, you can be sure I stop going. Tipping is the only etiquette that's hard to get.