I think it's the same formula used by any other long lived enterprise. Great Service. Beyond a basic level of technical and artistic competence commensurate with your pricing structure, what wedding clients will remember is the atmosphere of the day (or the absence of anxiety and concern for the photographers' conduct), the agreeable "flow of events" initiated by the photographers' instructions, and the photographers' people-management skills (some clients need bossing around, others not so much). This is for the day of the wedding.
There's is also the front-end consult and back-end post-shoot relationship, which I'm not qualified to comment on as much. Suffice it to say that they're just as crucial; these 3 aspects are closely interrelated.
Wedding clients won't remember (or care to know) how much your gear costs, or what f-stop was used for the portrait session. They'll remember the feeling, of how they enjoyed themselves that day. This is what leads to referrals. And longevity in this business.