Writing a Series that Holds Your Interest

I continue to add to my list of ideas for "Keeping a Series Alive and Lively" (an earlier version appeared as a guest piece on Chris Redding's blog).

Other authors write such wonderful mystery series it seems almost arrogant to create one, but I wanted one that had a clear protagonist as well as a couple of good friends and a humorous take on life.  It took me years to develop the setting (I love East Coast beaches) and characters for the Jolie Gentil cozy mystery series and write the first two books.  I kept moving between the two, trying to decide who would play what role and how they would work together.  If I was to get lucky enough to work with the characters over a long period they had to hold my interest as much as a reader's.

Consider these points as food for thought for a cozy series.

1. Will your characters maintain the same characteristics and perspectives throughout, or will they evolve?  Readers may not expect epiphanies from a light mystery, but they could lose interest if main characters repeat the same mistakes or remain stuck in a dull job.
2. Even-keel characters aren't all that interesting, but you're going to work with the characters for a long time.  Consider if you want to spend time a lot of time with a hot head, heavy drinker, or practical joker.
3. Will you get bored if your protagonist works as a real estate agent or librarian?  Maybe you'd rather hang out with a woman who explores shipwrecks or a man who creates components for the next U.S. forage into space.  You'll have to do research on their profession no matter what it is.
4. Will your characters move around the country or globe or stay firmly rooted in their home town?  If they stay in the same town you can introduce characters in one book and have them play a bigger role in the next.  On the other hand, unless it's a really large city the consistent setting can limit how much trouble your characters get into.
5. If there is something to learn in each book it can pique your own as well as reader interest. The challenge is to have new material without sounding as if you're writing an encyclopedia article. 
6. Is there a love interest?  With books and television shows, if your protagonist enters a committed relationship or marries it changes more than how they interact with others.  There is no longer "relationship tension," plus they have to keep someone apprised of their whereabouts and respond to the partner's interests and needs.
7. If you want to express a point of view -- political, religious, cultural -- consider writing an editorial.  If a particular opinion or piece of information is not integral to the plot or character it adds nothing and can sound like a sermon. 
8. Will your characters age?  Not only would your protagonist age, so would those around them.  If you don't want a favorite uncle to die, he either has to start out younger or live to be really old. 
9. How will your protagonist find time to solve a mystery?  Trust fund families are rare, and your hero can't be tied to a desk.  I made Jolie Gentil a real estate appraiser, which gives her some flexibility and has her deal regularly with different people.  
10. There are only so many dead bodies that appear in our lives, and there needs to be a reason that your protagonist runs into more than her share.  Or maybe there doesn't.  It may be enough that each book has a good reason for encountering one.

I'll keep adding to this list, and would welcome your ideas.