These are hard to do. Mostly because the author gets tired of them or the reader does, and they are hard to keep fresh.
The exception, of course, is J.D. Robb's futuristic mysteries, the "In Death" series. She's kept it entertaining, and if you haven't listened to these on audio, they're great. I think the books are into the late 20s now, and naturally I like some stories better than others. I think some are stronger than others, and one I've read only once.
The reason this series continues to work is the conflicts/opposites between the main characters, a very tough female cop and her very smooth zillionaire husband -- who made his money with brilliance and, early in his life, sticky fingers.
Eve, the heroine, is the driving force of these books, and both protagonists have a deeply troubled past...that Eve doesn't always remember (especially in the first books), so the revelations of their pasts and the clash and mixture of their lives, and the mysteries-suspense have enough going to keep the reader engaged over many books. For that, the pasts must be complicated, deep and troublesome.
The ensemble cast is excellent, too. The sidekicks and friends and the people she meets during her work.
But this can also lead to the main problem I think she might have as a writer, and that is bringing favorites onto the scene in every book. Most are folk connected with the NY police and security department, so it isn't too hard, but some aren't and including them in each book may be difficult.
The books take up a little more than three years "book time," so the relationship is still new.
I also enjoy the Sookie Stackhouse books, by Charlaine Harris, more people come and go in those books (and there are several "in the middle" that I haven't read). This series has more of a continuing arc that can hook a person, and it is set in Louisiana before and after Katrina.
My own series I consider "spin offs" with one couple a book. So, what keeps these going?
1) Sympathetic characters with enough depth in their pasts and personal problems in the present.
2) The characters clashes with each other, what underlying opposite philosophies will continue to clash and be explored.
3) A continuing arc (either personal in the Robb books or personal/plot in the Harris books).
4) Keeping the time period short.
5) Excellent secondary characters (though that might fall under number 1).
6) Heroes (male) that women want in their own lives.
7) The author still likes her characters and world.
Feel free to add, time is pressing me here. But I would say writing such long series (I am contemplating a trilogy with the same characters and that is challenging for me) works best in the hands of a master.
May you enjoy any/all the characters you meet today.