This is a tricky question because "best seller" doesn't necessarily mean "best written novel.'
But be that as it may, I think you have two kinds. There's your best seller, and then there's your runaway best seller. The ordinary variety of best seller is created by the unsolicited hype of several New York critics.
If you've managed to pique the interest of certain key people at publications like the New York Times, the New Yorker, and other such periodicals containing the words, "new" and "york" in their banner, then you can be sure to have a best seller on your hands.
Of course, to acheive that goal, you need to be hip, in a very ripe sort of coctail party way. Which of course means absolutely nothing. But it doesn't hurt to be well connected... at least as long as your connections stay connected. And as we all know, such things are subject to change without notice.
Well, okay, I'll level with you. It's all run by a small cabal of insanely rich Howard Hughes types who spend all afternoon in states of drug induced catatonia, burbling an occasional word or phrase. Most of it is unintelligible, but their underlings have a special system, based on the Kabbalah, which can turn the meaningless babble into something resembling dogma.
Like tachyons in a particle accelerator, each little literary dogma-zon only lasts for a fleeting instant, but if you are lucky enough to have something in the slush pile that vaguely relates to one, while it is in existance, you stand a good chance of being rejected. Which is much better than being ignored. Just ask any New Yorker.
And that's all there is to it!
Oh, yeah, and if you want to write a runaway best seller, just latch on to urban legend and aim for a folk audience. Never fails.