source: nownovel

• know what makes a series. include multi-book continuity and development.
• choose a series-wide conflict. what unknown could sustain curiosity across sequels?
• create an addictive world. add fun details that make settings worth revisiting.
• outline in advance. try to keep a bird's eye view of your series' arc for clarity.
• reveal characters gradually. build characters over multiple books and delay some backstory.
• introduce new characters. add new characters who help or hinder in sequels.
• give each character an arc. who wants what? how will they act on their desires?
• give each book a focus. be it discovering a world or something else.
• give strong endings and titles. resolve main conflicts and find titles that echo events.