Anyways, like I said, I'm happy to impart my very anecdotal advice about getting the back after obtaining an appointment, as I believe our playing fields are level from that point onward. Besides, even if I got my appointments the old fashioned way, it wouldn't matter. It's ultimately a computer process, no one here really knows how or why they're selected, and any guess at it is useless anyways. I think there's still useful information to give away.. FWIW, my Birkin was scored by obtaining an appointment by the regular way, and with a purchase history (at that store) of just a few shoes.
1) I've seen a lot of advice about "dressing the part." I think that's a bit silly. Yes, you should dress well (note: that doesn't mean just wear brand name bling all over your body). But many SAs have told me that they're trying to avoid selling K/Bs to people that might go right around and resell. I find the argument a bit silly, because I know many K/B collectors who routinely place their bags for sale and buy new ones. So, dress well, don't look shady.
2) Be nice. The store is filled to the brim with entitled shoppers throwing a fit that they can't get a K/B, especially one they want. And the SAs roll their eyes at it every time because they see it happen every day, and probably every hour. I'd suggest getting to know the person you're working with--who they are, how long they've been with Hermes, etc.
3) There are definitely ways to get banned from the store. If you're in the store to buy leather goods and you don't have an appointment (or aren't trying to get an appointment), you will be banned. You are definitely being watched the moment you walk in.
4) I would say that having the opportunity to buy a K/B is based on building a brand history. But my B was obtained after purchasing very little from the store (and it's unclear if one caused the other). While I had spent in the low 4 figures, which is high, I assume it's low relative to the other store's clients.
That said, being a fan and purchaser of the many other Hermes Products is a huge deal. But this isn't an Hermes mystery. This applies to any other highly sought after good, like Patek "Nautenauts"--Buy other stuff, and you'll have a higher chance of getting other things you want. By the time my second Kelly had rolled around, I'd already purchased $20k of goods from the store. To be clear, I buy only what I want.
5) I've seen a lot of advice in this thread about not being too particular in your initial request. The SA apparently just takes the request literally if you ask for a Bleu Electrique Togo Birkin, and when they don't find one, they bring no other Birkin back and you leave the store empty handed. This really depends on your SA, IMO. When I got my first Kelly, I said I wanted a Kelly in togo/clemence in a bold color. The bag I was offered was literally none of those things.
I do have a "hit list" of colors I want, size ranges, and leathers. So I do make open requests, but I don't think that's necessarily the key to the game here.
6) Finally, the restriction that the K/B (and C) can be purchased only once over a certain time period ended up applying to me. But I'll note that many in this forum have had different experiences. I read one post where a poster got one bag because her request was at the end of one period, and another bag the next week, because her request was at the beginning of the second period. I tried to play a similar game to zero success. I do believe the rule is applied arbitrarily.
All in all, my the most consistent advice I have is that YMMV, but that if you want to significantly higher chance of getting the bag, build a good purchase history at the FSH store. Don't only go to the store with an intent to get a K/B. That'll turn everyone off. And be nice and build relationships. I'm sure I'll hit another Kelly thirst when I reach 30 and the relationships I've worked hard to build will I think serve me well. This doesn't mean radio silence with them until I want another bag. This does mean that I'll make an effort to see them when I'm in Paris, ask them how they're doing, and shop for other Hermes items.
Edit: Also, timing. I've been told by SAs not to try for a bag during August or the PFWs. But I'll add that I've personally had my most success during PFWs *shrug*
Edited: PT Not needed