Das Fugen-S im Deutschen

Maybe many of you learned about composite substantives in German, such as Baumkrone, Bahngleis, Freizeit etc.
Now, in some of them you need a so called “Fugen-S” to connect them, but in other words you don’t use them.
I think that most of us asked ourselves: Is there a general grammar rule when to use the “S” and when not?
The great answer is: No, there is no such rule.
But this article from Spiegel-Online can give you some ideas about it: