The password system is easy: I write it on the envelope, along with POSTE RESTANTE, or, as it is called in German, POSTLAGERND. Example:
Kennwort: Winter
POSTLAGERND
Postfiliale 508
Mainzer Landstr. 250
60326 Frankfurt am Main
The letter will be stored at the post office for around 7 days. We can only guess when it arrives, but it usually takes 2 days for mail to travel through Germany, and I'll let you know when I've sent it. Then, you can simply walk up to the counter, put on your friendliest face and say:
>Good Morning, I would like to pick up a letter that is being stored here for me, the password written on it is "Winter".
The clerk will then pull it from some drawer and hand it over to you. No questions asked, no ID, nothing.
Important: You should first check on deutschepost.de/de/s/standorte.html whether your local post office allows this service ("Post lagern lassen"). Normally, they do. Only small branch offices that are integrated into other shops, like kiosks, don't.
However, I should mention that this system is rarely used, which in itself causes problems occasionally. Younger employees may not have heard of it. Once in a while, a letter is thrown away. Or the attractive young female postal clerks giggle all day long in expectation of the mysterious stranger who will come in to pick up this extremely unusual letter. In other words: Be mentally prepared that they screw up. Based on experience and feedback from Bernd, it works flawlessly around 70% of the time. Then there's 20% pls-wait-I-have-to-ask-my-older-colleague-what-this-means and 10% complete fail.
Bernds located outside Germany may use this system as well. However, an ID is needed to pick up the letter, hence your real name is required. Can't say why Germany is the sole exception in that regard. At least you can hide your address though.