Even though not all my books are in a series, I like how I can think about the next book as I write a book in a series and drop in a little nugget that gives the reader an idea of what the next book in the series might be about. It works well for the Shandra Higheagle books. I either leave a nugget about Shandra and Ryan's relationship or the possibility of who might be involved in the next murder that happens.
The same goes for series that are set in the same area or with the same ongoing secondary characters. While writing a book, I can think of a good premise for another book. It may not always be the next book, it could be one or two down the line, but I jot down the information and use it when I'm ready for that book.
Right now as I'm writing the first book of the Silver Dollar Saloon series and the secondary characters are coming to life, I can visualize the type of man each saloon girl will have for her hero, and I'm getting a good picture of the woman who will capture the heart of saloon owner, Beau Gentry. But he won't have his happy-ever-after until about book four. Why? Because the reader needs to get to know him better before they see the woman he falls for. ;)
Not only am I seeing the books that will come from the Silver Dollar Saloon series, I'm planning the next mystery and a contemporary western Christmas novella.
And the contemporary western Christmas novella. I'm still puzzling that out. I'd like it to have something to do with the National Finals Rodeo because it happens in December but at the same time, I'd like to write a story that sticks to what I know, cattle. So it may be a story set on a cattle ranch. Any preferences?
That is how my brain works. Constantly flipping story ideas and characters around in my head, assessing whether they will work and if I can pull off what I'm aiming for in the story.
As I've heard several times from friends and family after I've talked about the process of writing a book, "I never realized how much goes into a story other than writing it."