I have several things I use/do when I begin to write a sequel.
Re-reading the books in the series
The first, and most important, is to re-read the previous book. In the case of Kissing Alex (BG6) I re-read all five of the previous books! Re-reading isn't about the eye colours and the hair styles, it's about the feel of the book, about the rhythm of the story, the shape of it, and the style of writing. It's about re-learning the world that the book is set in, and making sure you get a handle on the rules of that world (BGs, UK, no guns, Montana, horses & ranch etc)
Notes and Maps
The first thing I do when I start a series is draw a map of the town/offices or a family tree, or i collect pictures that inspire. Then I make a storyboard of sorts. The one for the Texas series started off really neat and then disintegrated into scribbled notes everywhere!
I also like to give everyone a birth date, a height, eye colour and hair colour. This was always a tricky one for me. If you want to use a throwaway line like *He leaned down* or *his blue gaze held so many secrets* then you need to make sure that the actions and description matches through the books. In Texas Riley has hazel eyes and blond hair, Jack has dark hair and piercing blue eyes… and yes, that is possibly what people wouldn't expect (normally blond hair, blue eyes etc), so it's important to keep notes I can refer to.
So, I mentioned about dates etc. I found a piece of software called Aeon Timeline, which I use to track ages, dates of events, etc. This is vital because I was the WORST at recalling when things had happened, and how long passed between events, because I was so caught up in the narrative itself.
I blogged a bit about this software on another post here: Aeon Timeline
So, the answer to Kendra's question, is that I use a lot of tools to make sure the next book in the series is correctly timed, with cohesive characters…
You should have seen the notes I wrote for Montana 2!!!!!