Now: to the questions!
Anonymous @ 10:17 asks: "Are there just a few accounts that represent a large portion of sales? To B&N, say, and Borders? Or are those handled regionally? What does a salesperson -do- all day? I've never noticed one visiting an account, and I've spend thousands of hours in bookstores."
The major accounts are Barnes & Noble, Borders, and Amazon, although Books-a-Million and the wholesalers (e.g. Costco) move a considerable amount of stock as well. Sales to national accounts aren't handled regionally—the sales reps at the trade houses generally travel to their corporate offices to sell to the buyers—but there are field reps who handle a lot of the other outlets (especially the larger indie stores) and those are handled regionally.
As for what we do all day, I shall direct you to my recent account of the continuing adventures of INTREPID SALES ASSISTANT. And we sales folk do often visit the stores; we just don't usually do so wearing t-shirts that announce the publishing houses we work for, so you probably wouldn't spot us walking around. We are among you and LOOK JUST LIKE YOU OMG
Anonymous @ 11:15 asks for "Something on the seasonal swings in publishing; are there better months than others to have a book published, and do some genres do better at certain times of year than others?"
In my (admittedly limited) experience, February's not a super great month—there are a lot of returns from the end of the previous year and a lot of people are probably still plowing through the gajillions of books they got for Christmas. (Come on, I can dream, can't I?) The summer is usually pretty good, especially with all those movie tie-ins floating around, and the fall/holiday season is where we do the majority of our business.
In terms of genre/category—the only thing I can think of is that children's/YA usually sees a small sales bump around Easter that doesn't exist in adult sales, but that's really it. There's not really a "best month" for science fiction, chick lit, &c.
Kathe asks: "Can you please explain what constitutes a best seller? Does it vary by genre? I.e., YA versus Mystery versus Literary. Also when is the best time of year for agents to try pitching debut authors, especially YA?"
Alack, the exact processes by which bestseller lists (e.g. the New York Times' bestseller list) are generated are generally considered trade secrets and thus are either unknown to me or are not for me to divulge. Quite literally and unhelpfully, a bestselling title is defined as a title that has sold enough copies to be in the top x titles for that week, where x = however many titles the list in question feels like tracking. I imagine most lists are based on projected as well as actual sales, since these lists are available unbelievably quickly after new books go on sale each Tuesday.
As for Anonymous @ 4:35's question regarding how many copies it takes to make the New York Times' list: it depends on whether you're writing fiction, nonfiction, graphic novels, self-help, &c, but you'd probably have to sell in the tens of thousands your first week to land near (or at) the top of the list.
Bruce Pollock and Bron ask about the mythical "pub date" and if there's anything you can do if you realize you're pubbing on the same date as Oprah, Dan Brown, &c. The pub date is very simply the date your book goes on sale, and if it doesn't take off by word-of-mouth, land on a bestseller list, &c, there's probably not a whole lot more for you as an author to do for said book (at least in the way of publicity and things like that) after that date. And, alack, if you're busting into book stores on the same day as Hotshot Author with his/her brand-new book, Much-Anticipated Title With Initial Print Run in the Millions, there's really nothing you can do about it unless you also are some kind of hotshot author with a major book. The publishers will move your pub date if they think you're important enough; they won't if they don't.
And, finally, to Anonymous @ 4:21 on 9/10: foreign sales are a very tricky issue that I don't think I'm entirely qualified to blog about, but if a book sells phenomenally in another country, the publisher will probably decide to continue to only publish that author in that country rather than drop him or her from the list altogether (a strategy they may re-evaluate a few books down the line once enough reviewers in, say, the US have said, "Whoa, check it out, this author's books have been selling super well in Germany, American readers should be reading this guy again," &c).
Tomorrow: Dan Brown kills an entire forest!