I've written a five-book arc for the Starcaster series. Ambitious? Nah!
Anyway, I've come up with some simple themes and decided what I want to do with them in each novel. For example, I've come up with a Career Arc and a Romance Arc for Tory, along with a Politics Arc for the political situation of her world. These arcs seemed to suggest certain titles. Therefore, I've named all five of my novels, and I have written a one-paragraph synopsis of what each one is about.
For each arc, I try to do something different in each novel. For example, in the Career Arc, I've decided that Tory needs to spend some time as an illegal spy. She's an outlaw spy for most of the first book, but for the third book, she's not a spy at all, but ends up spying anyway. In the Politics Arc, the benevolent king gradually turns paranoid over the progression of the novels, and becomes repressive toward the end. As he grows more repressive, a certain organization grows more powerful.
I think five books is enough to plan out for an unsold series. When (think positive!) those five are published, I'll see if I want to continue it through books 6 thru 10, or if I'll just give Tory a Happily Ever After and let her retire.
Just doing all this while I relax during my post-draft slowdown. I imagine I'll be back to serious polishing by the weekend.