"Every piece of invented information that’s in that bad-boy-of-a-series is coming from your brain. It’s like this giant sprawling 50,000-piece puzzle of your own personal design; everything’s gotta fit together. And when you do sit down to try and put this puzzle together, it typically doesn’t happen overnight."
--Tiyana Marie White
It is particularly important to develop the story and characters slowly in a Series.
Your characters arcs must develop in an incredible amount of pages, so make your heroes grow slowly. Take your time, but make each change count.
Develop a series of conflicts and keep a few key conflicts unresolved in order to be able to develop them in the following books.
There is a world around the main book and initial plot that emerges in subsequent books. The sequel unveils what was there in the first book but was hidden from view or put aside in order to focus on the story of the main characters.
In Twilight, from the beginning the key subplots have been the werewolves and their role in Bella's world and Bella's friendship with Jacob.
In book 2, Jacob's world emerges.
At the end of book 1, Victoria's subplot emerges and will be present in book 2 and book 3 of the sequel.
Pit your characters against incredible daunting odds in each book. Do not rehash the same old conflict. And make sure to invent new twists and dig up startling pieces of information. Usually the lovers in Paranormal Fantasy never get to consummate their love, except in the last book of the Series.
In Vampire Academy, The love story between Rose and Dimitri is barred by an increasing number of walls.
In book 1, Dimitri is Rose's teacher. He is much older, and she is not yet 18, so they must keep at a distance because of propriety.
In book 2, Adrian becomes a rival to Dimitri.
In book 3, Dimitri becomes a strigoi, the enemy of Rose.
In book 4, Dimitri does not think he deserves Rose. He must fight his own ghosts and guilt for killing innocents.
In book 5, Rose and Adrian's romance has evolved and Dimitri does not want to take another man's girlfriend.
Each minor character needs to be able to have his or her own story. Do not weigh down the heroes with relatives to take care of, or kids, pets, or physical disabilities, or with anything that needs to appear in each chapter but are more of a burden along the way than a help to develop the story.
Insert enormous twists in each new book. You need to surprise the reader. For example, enemies become allies, friends become enemies, a gift becomes a flaw, a love interest falls in love with somebody else, etc.
In The Girl of Fire and Thorns, Elisa is kidnapped by her enemy. On the way, they become best friends.
In each successful novel sequel I have read, a spell/issue/event keeps the heroes apart. You need to find a way to keep the heroes/lovers apart in order to be able to develop subplots and make the subplots come up to the surface. The separation should give one of the heroes the incentive to go find the person they love and confront danger to change/save them. The separation usually occurs in book 2, but it could also occur in book 3.
In Match, the hero is sent to a distant territory and it is up to the heroine to find him. The absence gives her a chance to deal with her best friend and people in charge of the match.
In a lot of novels, there is a love triangle that serves only the purpose of keeping the heroes apart while dreaming of each other and exacerbating their love and feeling of jealousy.
In the Mammoth Hunters, Ayla meets a new tribe, the Mamutoi, for the first time and she gets confused about the cultures and rules. Jondalar gets terribly jealous when Ayla sleeps with another man, and, as a result, they become distant. We need Jondolar to step away for a moment for Ayla to discover a new culture on her own without a mentor.
Make sure the central conflict of each book is solid and is resolved at the end of the book.
Introduce new aspects of the world, the characters, etc. Introduce new stakes.