In a trilogy, book1 is the tip of the iceberg. This means that in order to have a great trilogy, you need to be prepared and get the general plot for the sequels already planned.
Some authors wait until they see how book 1 is received before they draft the sequels, which means there will be a lot of missing parts in book 1 and the sequels might not feel as consistent.
Here is what to watch for:
Decide how your story will end. You need to know because you need to decide which major obstacles and major changes your protagonist needs to go through. Each sequel should show a clear progression, but you, as author, will be the only one to know about it.
Use foreshadowing. Hint at places, objects, people who might have secrets or that might hide important clues. Ideally, book 1 should contain all the elements you need to build the sequels. At least book 1 should hint at the remote cousin who has a grudge or the portal under the sea no one has ever found.
Have a list of twists you want to introduce in your trilogy. You need to weave the story elements to prepare the reader for the twist. Sometimes you need to prepare the reader from book 1 for the twist in book3.
Even if you do not want to invest time in a trilogy before you see book 1 published, at least draw a probable sequel’s map in case you decide to add something to the story later.
Consider the backstory that will come to play an important role in the series. Think of what the characters did in the past that will shape the future, the people they know, the alliances or enemies they formed, the people they left behind, the mistake they’ve made, etc. Slowly drop a few bombs from the past on the way.
Keep the same tone for each book. Stay in the same genre and stay as consistent as possible with the themes.
Plot your timeline carefully. Do not freeze your characters in time, but make sure the time passing is realistic and believable. Time is of the essence, especially when years are passing where new inventions/ fashions/aps. emerge (from book 1 to book 3), when weeks and seasons change the scenery and the hero’s mood.
Some YA authors decide to write a book for each year of the life of the hero.
Hill, Beth. “Setting up a Series.”
Britton, Vickie and Loretta, Jackson. “Planning the Series Novel.”