This is a loaded question, and one that can have a variety of answers. How passionate is the author about the topic/subject? Are they able to write in an engaging manner? Is this book unique or are there 3 others just like it already in stores? Assuming the answers to these questions are favorable, there are typically three key ingredients for success in any type of publishing: platform, placement and promotion.
By "platform", I mean author platform, primarily. Years ago, this implied things like author visibility, speaking engagements, whether they're a regular columnist in a key magazine, etc. These are still important pieces, but now you have to add in things like how popular their blog is, how large an e-mail list they have access to, etc. That's one of the reasons why I'm so excited to be part of the team that's going to publish Robert Scoble's and Shel Israel's upcoming book on blogging. These guys have an enormous platform which will help drive sales of the book.
The placement component rests on the shoulders of the publisher. You obviously can't sell a copy of a book if the store doesn't have it, but ensuring there's at least one copy on the shelf is only part of the battle. You need to devise a plan that helps this book rise above the shelf noise created by all the other books. That's where placement's close cousin, promotion, comes into play.
Promotion is often the trickiest piece of the puzzle. I've seen far too many cases where promotional dollars were spent trying to increase awareness, only to result in a disappointingly modest increase in sales. Authors often push their publishers to spend a load of money on ads in trade magazines and newspapers, for example. I'd rather spend that same money on a promotion with one of the key booksellers. Remember my earlier point about rising above the shelf noise? The best way to win this battle is to put a book in one of the highly coveted promotional slots in the store. This results in placement on an endcap, in the aisle, by the cash register, etc., along with a nice discount to the customer (typically 20-40% off) -- now that's where you start to move the sales needle. A full-page ad in USA Today is great, but wouldn't you rather catch that same customer in the store with their wallet in hand, ready to close the transaction?
If you're a published author, don't go beat up your editor because your book never made it into an in-store promotion. Only a small number of books can be promoted like this at any time. Most promotional decisions are jointly made by the publisher and the buyer who works for the store/chain. Lots of factors go into this, and maybe that's a good subject for a future blog entry... Btw, although this seems to focus exclusively on brick-and-mortar sales, similar promotional techniques work for the online resellers: cover placement on a subject page at Amazon, their "Better Together" promotions (where they encourage you to buy two related books rather than just one) or inclusion in one of their e-mail blasts can have a huge impact on sales.
I wrote about author platform first because I feel it's the single most important ingredient for a bestseller. Sure, placement and promotional activity can boost sales of just about any book. But the sales difference between a book written by an author with a solid platform vs. one with a lesser or no platform is huge. Thanks to all the platform vehicles available these days, authors can help drive more sales than ever before.
If you're already an author or just thinking about writing a book, take the time to develop an inventory of your own author platform. Just because you don't have a website with tons of traffic or an e-mail list with a million names doesn't mean you have nothing to offer. Are there any contacts in your address book who do have access to these things? If so, how can you leverage that relationship? The more tangible platform components you bring to the table, the greater the likelihood of getting an editor's attention -- more importantly, the greater the likelihood of becoming a bestseller.