As you have probably gathered, Germans tend to believe strongly in the concept of a right place and right time for everything. Which also implies a wrong place and wrong time. For many people here, a wrong time to phone someone at home is after 10 p.m. Newcomers will often get unexpected nasty responses when they make this gaffe. If you do want to call someone after that hour, ask beforehand. Some Germans have no problem receiving calls up ‘til midnight, but do check about this first.
Likewise, try not to simply drop in on people, even friends, unexpectedly. This practice which still survives in many cultures as a sign of affection and trust is seen as an imposition here. One exception is if there is some real emergency, and these friends or acquaintances are the only ones who can help. But that notion of “We were here in the neighborhood, so we thought we would…” is not very popular here.
Another point about the right time: Try not to give a birthday present before the actual day, as that is considered to bring bad luck. If you can’t be there, it’s better to give your present after the day. This applies for both friends and business colleagues.
While on the subject of birthdays, again be aware that the birthday girl or boy is responsible for throwing their own party. In the private realm, this usually simply entails inviting a group of friends along to a café or pub and treating them to drinks and a light snack. More important birthdays sometimes call for taking the group to a nice restaurant. While you should certainly expect a gift from everyone there, you’re expected to pick up the tab for the celebration.
The above primarily addresses the question of how to maintain good relations with people you’re already on a friendly or social basis with. There are other things that are important for people that you have just met for the first time, such as in a business context.
Always look people right in the eye, be it your boss, a subordinate or an executive at the company you’re doing business with. The deferential looking down when meeting people of a higher rank that works so well in many cultures is viewed negatively here. Some might even take it as a sign of dishonesty.
Speaking of dishonesty, be sure you’re being sincere when giving compliments. Germans are like everyone else, they love getting compliments. But they’re also unusually attuned to what they feel is false flattery. Mild, friendly exaggeration is fine, but too much of it and people may start to distrust you in general.
One further tip on making someone’s acquaintance here, be it in a social or a business context: Most Germans appreciate a firm handshake on introduction, especially from males. It is not uncommon in the business meeting to shake everyone’s hand at the table. And that deferential limp handshake that some cultures favor does not make a very good impression here.
Remember that there are two sides to etiquette, and just as you may do something that will inadvertently offend Germans, some German behavior might rub you up the wrong way. So the best thing you can do is prepare yourself for this and expect it as part of the experience of being abroad.