1) What you establish in Book 1 you have to live with for the entire series.
2) If you need to get out of what you’ve established in Book 1, your readers will know you’re cheating.
3) Series work best if the characters evolve.
4) Evolution does not mean a sudden radical departure from established characteristics. That’s cheating, too.
5) Technology will change faster than you can write your next installment so avoid too much technological detail in your story.
6) Your brain is not a bible. Start one early.
6.5) Series are the underdog of literature. You’ll have to work extra hard to earn respect.