Before I start, let me give one caveat: This is my take on things based on observation and asking pesky questions, not a scientific study!
I've gotten to a point in my career where I've been hitting bestseller lists and many of my writer friends (at about the same point as me) are too. It's kind of fun to sit back and watch which books hit, and I've come to realize what makes a bestseller.
Five things make a book hit the top lists (USA Today, Pub. Weekly, and NY Times) upon publication:
1. A large print run
2. Excellent distribution with reorders filled quickly
3. Placement in stores
5. A great book on a subject readers care about.
Now, if you look at each of those, which do writers have most control over?
Number 5 of course.
(Number 4 a little bit, but when I say "marketing" I'm talking about to distributors and booksellers, which is done long before the book is due out.)
Looking at these in turn:
1. A large print run: A good sized print run (say 65K and up) will allow you to hit USA Today if all the other factors are in place. Bigger ones let you reach higher. Let's face it, the more books out there, the more available for purchase.
2. Excellent distribution with reorders quickly filled: If no one can find the book, no one can buy it. Simple as that. The largest chunk of books are sold in Walmart, Target, Costco, drug and grocery stores, and other retail giants. Surprisingly, bookstore chains order only a small percentage of the print run. Independent bookstores are also a small percent, but Indies are good at selling! (Because they're usually owned and operated by people who truly care about books. Go Indies!)
You want the sold books replaced quickly because, duh, if the book's not on the shelf, no one can buy it (online sales can't take up the slack at this point). Good distribution where the book stays on the shelf=good sales.
3. Placement: Getting your book on an end cap, a table, face-out on a shelf, or getting a shelf-talker (those labels of the author's name on the shelf itself) help readers find the books. Placement is not up to you the author, it's up to the bookseller, and what the publisher negotiates with the bookseller. Publishers can "buy" good visual space for a book they want to push.
4: Marketing: I mean the sales reps for the publisher enthusiastically selling the books to the distributors or bookstore buyers. The more belief a publisher has in an author or title, the more enthusiastic the reps are, and the more interest the buyers have.
Author marketing can help too. If the bookseller knows you're out there with a video, bookmarks, a website, a blog, a newsletter, etc., that's a good thing.
5. A darn good book that readers want to read: Ok, writers, this one's up to you! :-) A book with a strong voice and strong characters, well plotted and well constructed, on a topic readers care about is what you need to do. (Easy, right? [snicker]). Keep trying (I am).
When I say "what readers want to read" or "a topic readers care about" I don't mean the latest paranormal trend (though catching a rising trend helps). I mean a theme like healing, coming home, finding one's self through love of another--all those universal constants that appeal to us.
And, true, a popular time period and setting doesn't hurt. It's sad but true that certain times/places sell better than others. (This varies from genre to genre, so study your genre before you start!)
Ta da! A bestseller.
I know this is simplistic. I always need to break things down into simple terms to understand them myself.
If all of these factors are in place, there's still no guarantee the book will sell well. But it's a very good possibility that it will.
There are things that can weaken the above setup:
Books being sold early, so the first week's sales are not as strong (because of the dribbles before). There's a reason J.K Rowling's publishers were trying to sue people selling the last HP book early, and not just because they might give the plot away. They wanted that huge, frantic burst of sales on day one to guarantee that it was number 1 on every list.
The print run selling out quickly and not being replaced fast enough, or at all. (No books on shelves=no sales). Interestingly, some publishers just will not reprint a book. They decide--the print run's gone, we're done. I'm sure there's some accounting reason for this--don't ask me.
A stronger selling book than yours making booksellers adjust what's on the end caps, tables, etc.
Sometimes readers just don't connect with the book, even when you've done your job. If they don't like it, they stop buying.
But enough depressing stuff. Go out and write a strong, well-plotted, well-characterized appealing book :-) If the publisher likes it enough, they'll push it--and if they get factors 1 through 4 in place, you might be on your way to bestseller-dom.