A very basic guide, since I'm in an expensive computer class (that Uncle Sam is paying for) class and should be paying attention...
Background: I'm an American Soldier (soon to be Government Service (GS) employee), living in Germany for almost 3 years now. I've become rather disillusioned with Americans and their behaviour. Far from being un-Patriotic, I just think our attitudes need to change.
1) Leave your SUV in the US.
I drive a BMW 525ix, which is a "large" German car, and I have enough trouble getting into parking spaces and city streets/corners. I have no idea what purpose you think you need your Lincoln Navigator in Germany for. It's too damn big, and uses too much gas. I have no idea why we're so damned obsessed with driving these wastes of space. (I see way too many people commuting by themselves in these boats) Forget it, buy an Opel or something. On that one day you plan on buying a couch from IKEA, you can rent a van for around 20Euro.
2) Stay out of the left lane.
Especially if you're from California, this is for you. The left lane is for passing only. Do not cruise the autobahn in the left lane, you will piss off a lot of people, and are likely to cause an accident. You will be ticketed by the Polizei for this behaviour as well. Also, look in your rear-view mirror, it's there for a purpose. If you see someone "flash" you, it's not because the want to say "hi", it's because their about to be up your tailpipe shortly if you don't move over.
3) Reacquaint yourself with "cash".
Not that Germans don't understand the concept of credit, they prefer not to use it. For the most part, all transactions are done in either cash, or using an EC "cash card" that is a smart card linked to a German bank account. Get used to carrying cash (this includes coins, since 1 & 2 Euro amounts are in coin form) Euros around if you plan on going shopping. (Or paying your ticket for cruising the left lane... Polizei do take fines on the spot.)
4) Learn basic German.
It's not that hard, and it'll help you get around, order food, buy things, and avoid confusion. Granted, a large percentage of Germans speak English, but it's a nice courtesy to at least try to speak German.
5) Restaurants are not like Olive Garden.
Eating out in Germany (Europe for that matter) is strikingly different than eating in the US. This section could get it's own outline, but I'll try to keep this short. Seat yourself unless someone is greeting you at the door. Don't sit at a table that has a "reserviert" placard on it. There are no free refills (ok, so McD's just recently started). Expect to pay the same (if not more) for a .5L glass of Coke as for a tall glass of bier (that's beer to you). Also, soda is different here as well. For one, it's not as carbonated. Also, it tastes different. Hard to describe without talking about every flavor, but for example, Sprite is very lemony (tart!) compared to the US version. Oh, and don't expect ice, it's not all that popular. If you want water, make sure you let them know "ohne gas, bitte" or you'll get the popular water that is carbonated. Second, don't expect the waitress/waiter to come by and take your order if you have your menu open. It's a sign that you're still deciding. After that, don't expect them to come by every 5 minutes. The general way it's done is you have to get their attention. Dinner is a relaxed, slow process that can take a couple hours. If you are ready to cash out, you need to ask for the check. (Quick German lesson: Say "Zahlen, bitte")
6) Don't leave the tip on the table.
Just don't. Hand it to the waitperson. A "Vielen Danke" would be nice. German waitpersons don't live off tips like Americans do, so you can tip less (10% of the bill) or simply round the bill up, saying the number i.e. 15 Euro when it's a 12,30Euro bill.
7) Calm down.
Americans are way too hyperactive, expecting everything "right now". For the most part, Europe is relaxed. Lunches take a while, usually followed by a walk. Expect to pay for customer service on the phone, it's just how it's done, quit complaining. Don't expect to get anything done after 4PM, after 1PM on Saturday, or at all on Sunday. Get over it, I'm tired of hearing you complain.
8) And lastly, for God's Sake, don't call anyone a "Nazi."
As ludicrous as it sounds, I've seen it happen. If you're an American, and I hear you do that, I personally will bitch-slap you before you can further embarrass the US.
I may have offended some, but I honestly don't care. I'm pretty pissed off at how a lot of us behave over here.