The first piece of 'long' fiction I wrote was a novella parody of Stephen King's 'Christine.' I was in high school, and my version was about a kid with a possessed locker instead of a possessed car. It was also my first attempt at humour, which fell completely flat because no one who read it realized it was a parody!
When a series is doing well, it's very tempting to keep writing it, even when the creative well is drying up. It's tempting because that's where the money is. I've had to be very careful; as soon as I think I'm getting close to that dry well, I wrap the series up. I don't want to just keep writing something because it sells.