Finish the first series before starting a second. Or a third. (note: I'm a fast writer but I take FOREVER in revision)
The third installment in my espionage thriller series is slated to land around the end of this year - three years after book #2. In between, I wrote a four part horror series (3 novels + 1 novelette) plus a SciFi novella plus a few other projects that languish on my hard drive in various states of completion. The horror series was supposed to be a standalone, to scratch an itch, but it sold pretty well, and I was still feeling fatigue from the first two books in the espionage series, so I kept on going.