I've given up never ending series, so only have a handful of in-progress-series to keep track of and only one where I actually buy the books still (one more book to go!), instead of borrowing them from the library.
The trick is not to start them. Otherwise you are stuck, unless the story gets really bad, but then you still want to know what happens in the end, even if the author's writing/plot gets silly.
I really do love a long series, though. Looking at that list, I just think I need to be a little bit more selective in my decision making.