Hey You! The tricky subject of the German "you"

Something that causes a lot of confusion for English speakers here is figuring out which form of "you" to use. The formal "Sie" or more casual "du". The rules go something like this...

If the other person is around your age or younger or a good friend then you can probably use "du" when referring to them. If the person is older, someone you don't know or someone deserving of respect then you should use the formal "Sie", unless they tell you it's ok to use "du".

And when you use "Sie" it also means you would refer to the person using their surname. Germans use their surnames a LOT. When I go to my hairdresser they always say "Guten Tag Frau X". Hello Mrs. X. It makes me feel a little old!

Normally if you meet someone and as you become friends they would probably ask "Können wir uns duzen?" Shall we refer to each other using the informal "you"?

At work, a lot of companies have a general "du" policy where employees refer to each other with the "Du" form. The boss may still be "Sie" though and any clients would naturally be the more polite and respectful "Sie". It's also considered an insult if you had been on "Du" terms with somebody and then you start referinig to them with "Sie".

I was also recently told by someone that you would use "Sie" during self-defence, which surprised me. I thought that if someone was hassling you then you would not feel like using the respectul "Sie" term but my German friend pointed out that anyone listening to the conversation would more likely come to your aid if you used "Sie" as then it would be clear that the person is a stranger to you. So you would say something like "Hauen Sie ab!" Get lost! or "Lassen Sie mich in Ruhe!" "Leave me alone!". Just something worth bearing in mind if you ever find yourself in that situation!

Some more tips and rules are outlined here.
Even Germans can get confused sometimes about which they should be using! So I wonder what hopes us non-native German speakers have of getting it right!