I just watched a program on Five TV on the Autobahns, a few official and unofficial rules were stated such as:

It's illegal to flash the driver in front if you want to overtake, you must leave your left indicators on to show you wish to pass.

If you see an Audi, BMW, Mercedes, Porsche - any of the German bunch you should move out of their way.

If you plan to travel over 100mph you should have your front foglights on.

Hand gestures will get you heavily fined.

Anybody know anymore, it's quite interesting to find out things like these.