To distil the well known anecdotes and casual conversations on the subject... :
Essentially it is all about not simply being a good customer these days but one of your dealers most profitable ones, and in possession of an excellent relationship with your salesman. People who trade in both their heavily optioned Turbo S and Cayennes every year for new ones and have been doing so forever, that kind of thing. It is also in the dealers interest if the people that they give the slot to aren't expected to keep it for too long and if it is sold that they can expect it to be traded in to that dealership (so they can sell the same car again for overs with little use). Have you noticed how plenty of used examples show up within 6-18 months of launch at OPCs with hilariously low mileages? Those are from the people that they sell them to.
aperbag:
In simple terms though, if you are asking the question after the car has already been publicly revealed then unless you are a major celebrity (or a clickbait social media flavour of the month) you aren't getting one. Before the launch, the dealers know who of their top customers wants one and of those: who is definitely getting one, who might get one if they get enough allocation and who ... isn't.
I've been driving GT3s for so long (over 12 years now - the only time I was without one was the couple of months it took me to buy a replacement for the one that I wrote off on the 'ring) that I used to get sales calls from my OPC when they launched a new one, but now (and yes - I did check the last time I was in there) it isn't even worth my asking the question. Being someone who will keep it, drive it and not flip it will not in itself have any meaningful influence on you getting a build slot. It is all about your relationship with your OPC.