Yeah, I've got a 15 book series myself I'm working on, but I found a rather ingenious way to keep the storyline fresh, by rebooting the entire series at two separate points, allowing it to start with a fresh set of characters and a new point on the timeline, more or less allowing me to start all over while continuing the actual parent story. Some people don't seem to do that and thus burn out along the way. Heck, I'm already finding myself a bit stretched for the last two books in the first saga, but I've already found a decently creative way to ramp things up still another level so as not to disappoint.
But yeah, I'll agree that it does get harder to stay original the farther you go into a long series. I also found that if you toss in a few mind benders along the way, it helps to mix things up a bit and keep the storyline original and inventive.