#5: Make 'em "Say something in German"
There is nothing worse than being put on the spot to blurt out some random German phrase for the amusement and entertainment of English speakers. At least give us something to translate. Even then it's awkward but at least we have a straw to hold on to. No matter what I would say otherwise, would please you. After I say the German phrase, Americans tend to be disappointed with my selection anyways. The only way to make your audience happy, is to include lots of throatal "ch" sound. That's really all they want to hear anyway. I always feel like some bizarre circus act, Hans the Bilingulist.
#4: Make 'em learn Geography
Almost every American has some sort of connection to Germany. A lot of members of the armed forces served over seas. They usually served at a base in some small German village nobody has ever heard of. Yet I am always expected to know all about it. Have I ever heard of Hinterstabholzingen? Probably not. So please don't look so sad when I don't know what you are talking about. What adds to the confusion is the fact some cities are just pronounced weirdly by English speakers, so maybe I do know the place you are talking about, but I have no idea what you are saying.
#3: Bring up Hitler, Nazis and the World Wars
Germans are taught to take anything Third Reich related very seriously. Whenever Nazis are brought up in Germany it usually is in a historical context that will lead to serious discussions. Therefore whenever Americans bring it up in joking matter it will make every German cringe.
#2: Mention Sauerkraut and Bratwurst a lot in conversation
Yes, Germans like sauerkraut and brats, but just because we eat that stuff, doesn't mean we want to talk about it 24/7. When I look at a vending machine trying to find a candy bar, I don't need snipe comments that I won't find any sauerkraut in there.
#1: Proclaim "German's love David Hasselhoff."
Yes, there was a time when The Hoff had a few hits in the Old Country. He sang "I've been looking for Freedom" during German Reunification. It fit at the time. If Germans had know that they'd be hassled about that for decades to come they would have gladly refused David's visa. Now we are stuck with a dark musical past and would like to move on, but you Americans won't let us. I looked it up on Youtube just to see how bad it really was, and it was even worse then I remember. So if you really want to annoy a German bring it up, but don't say I didn't warn you if you get a German evil stare.