This got dropped in my e-mail from Writers Digest. I thought it was pretty good.
I’m not that sure about number five, about writing for a market. Where do you draw the line? Where’s the border between “writing for the market” and “market conscious?”
“There’s a saying in publishing that the moment you spot a trend, it’s too late to join it.”
Maybe. But some trends seem to have a long half-life. Supernatural romance, often involving vampires and/or werewolves, doesn’t look like it’s run its course yet. Not my style, unfortunately. Girls and women in my books are more likely to pop you in the nose than get all moon-eyed over somebody with unusually pointy teeth.
I get a daily e-mail as part of my Publishers Marketplace subscription. It lists new deals for books. For a while, just for fun, I kept track of the new book deals that involved vampires in some way. I’d cut and paste them to a Word document. I got bored after the second page, but I got the idea that the trend was still pretty healthy.
A couple of days ago I was tracking Kindle best-sellers out of curiosity, and the “Fifty Shades of Grey” series occupied the top three slots. Haven’t read them, and don’t plan on it, but I looked up the author and background. I was surprised, but only a little, to find out that this series had its genesis as Twilight fan fiction.
TomAlanBrosz 
What they mean about it being to late to join the trend is that, as books take over a year to get published in the big houses–just from the point of the publisher offering an author a contract to it being available on the shelves–and if, in theory, you identify the trend and are inspired to write a similar book, thereby adding another year (or more) to the timeline in order to write the book, shop around for an agent, and then sell it…do you think the trends of today are still going to be popular in two years or more? I think people are going to get pretty tired of vampires by 2015, personally.
Yesterday, the Publishers Marketplace deal list included a YA book pitched as “The Hunger Games” meets “Game of Thrones.” Two marketing birds with one stone!
Of course it’s possible agents and publishers are cherry-picking existing market-oriented manuscripts that have been around a while, rather than authors writing new work specifically to the market. Considering the timeline issues you pointed out, this would make more sense.
In short, some authors are lucky enough to have written something that became a trend after they wrote it.