There's a lot to learn as you progress in writing a series. More often than not, the lessons don't become very clear until you're well into the second book. In my case, as the characters from "Of Murder and Monsters" are continuing to grow and develop, I found myself jumping back and forth between the current manuscript and my Kindle copy of Book One. This wasn't terribly annoying until the sixth or seventh time I had to jump back into Book One. The reason for all of the jumping back and forth was to make sure that not only was I staying consistent with the descriptions, but that character growth picked up where I left off. Without keeping a certain level of familiarity, you'll lose that sense of continuity that gives your reader a platform from which they will accept the characters' development.
As I've continued to make progress on Book Two, I've started creating character sheets. These actually aren't that dissimilar from the kinds of character sheets you might find on a table top role playing game. Role playing game character sheets can serve as a great template. Some are incredibly detailed and cover everything from physical descriptions to physical and mental attributes. Of course, you'll want to input words and descriptions, rather than numbers - unless that's what works best for you. I also created a "notes" section where I keep a bullet point running list of the major events of Book One that contributed to the character's growth, challenges they faced, or how they died. Likewise, these bullet points also note the page where this happened on my final draft (just in case I still need to go back). Not only did this put the information right at my fingertips, it also has saved me quite a bit of time. Now, I no longer have to remember specific pages or chapters where something pivotal in the character's life happened.
The other helpful thing about putting together the character sheets is it has allowed me to make a better side-by-side comparison of multiple characters. Most stories involve character interaction (i.e. dialogue). It's one thing to have an idea of the personalities at play, it's another thing to be able to sit down and analytically look at your characters' quirks and traits in black and white. The helpful thing about this is it forces me to see where a character might go against the grain of their personality - maybe they're not very confrontational, or perhaps they're giving something up to reach a much larger goal later in the story. My point is, I've found this to be good for brainstorming when it comes to having the characters work with, or against, each other.
Writers: what kinds of devices do you use to help maintain character continuity in your series? Readers: how important is character continuity to you?