The key to a continuing series is...

Good topic. I like it a lot.

I've done a few series in my time and I can say with complete sincerity that, until SEVEN FORGES< I have ever once completed a series to my satisfaction.

Seriously. first there's the Chris Corin Chronicles. POSSESSIONS, RABID GROWTH, (ROOTS) and the unnamed final book. ROOTS never saw print, but I'd plotted it out in my head. The publisher A) didn't want it, and b) went belly up.

Next up, the Brittany Corin Chronicles. NEWBIES, (TOWNIES) AND (SCHOOLIES.)

The publisher went belly up while we were still in discussion about a sequel.

SUBJECT SEVEN and SUBJECT SEVEN: RUN! More books slated, but low sales killed the series.

 Seeing a trend here?

So am I.

The thing is, you never know.  If I had to say any of those series were missing anything, I'd say it was evolution of character. Does that sound vague enough?

What do I mean" I mean Chris Corin was broken and badly. Eventually he would have mended assuming he survived the pulpy and extremely violent series.

Brittany Corin was broken too, but getting better. I think she had a better chance of evolving and becoming a character people we interested in watching change and adapt.

The SUBJECT SEVEN series involved several characters and there might have just plain been too many of them for a YA series. effectively there were 12 main characters. That's a great deal to juggle. That is also a weakness in a lot of my earlier stories (Okay and maybe in SVEN FORGES, but there's gradual expansion thee that was missing in earlier series): I mean, there are 187 named characters in the SERENITY FALLS trilogy, which spans three hundred years in a town's history.

I COULD call SERENITY FALLS a successful series, by the way, as it reached the conclusion I'd planned, but to be fair it was originally one very large book and at the request of the publisher I broke it into three and added 40,000 additional words to make, I think, a much better book by the time it was all said and done.

Know what SERENITY FALLS had? Evolution. Damn near no character in that book ended the same  way they started. The events in that tale changed them (often by killing them, granted) and not always for the best.

If you are looking for a key to a successful series I'd say that's a big one.

I'll throw you a bonus: Continuity. Do it right and a reader will follow you anywhere. Do it wrong and you will have a serious problem. What do I mean? I mean readers, as a rule, are smart. Make sure YOU remember the details of your plot and world or suffer the consequences of readers who shake their heads in disgust. One of my personal favorites? In a book I was thoroughly enjoying he main female lead went from being a dark haired, pale skinned lady to being blown haired and blue eyes with freckles. Then she went back to the original. As it was a post apocalyptic setting I think we can safely assume she wasn't; wearing any wigs or dying her hair.

I could also give you a dozen other suggestions, but, hey, where's the fun in that? I have five other prostituting scribes to follow me here. Let them have some fun, I say!