As a general roule, try to keep a low profile. Do not take pictures of people without first asking their permission. Do not descend on sights like a swarm. Tip the guide. Try to read up a little on what you are going to see beforehand, so you'll get more out of the tour. Do not expect the guide's English to be absolutely flawless. Be patient.
Unfortunately, the "typical" American tourist is viewed as loud and obnoxious by some German people. Try to avoid speaking in a loud voice when in public, especially in public transport. Try leaving your cell phone at the hotel when you go to a restaurant. Do not get intoxicated in public. Be aware that the German beer is stronger than the US versions.