Well, it's not really what Americans do on a daily basis back in the states that's considered rude, it's just most of the other Americans that I've seen over in Europe just act crazy and overly selfish for some reason and because of this attitude is why we have a bad name all over the world. No, not all of us Americans are a bunch of selfish, ignorant neandethals but sadly a lot of American tourists do act like apes when they're abroad and believe me you'll see them and they will embarres the hell out of you (so much so, a lot of times I pretened that I wasn't myself). Basically, here are the things to consider so that you don't repeat the sins of so many of our countrymen:
1) Don't talk with your friends so loud so that everyone can hear you: this draws attention, is just plain rude and disrupts everyone elses peace and quiet and attracts the scam artists and theives (who love to rip off American tourists) because they know who you are.
2) Don't get impatient, no matter what the situation, because in Europe they don't like to be in a hurry for everything and they don't like being told what to do, while in the US we don't seem to mind it. Also, just because something's not getting done real quickly, doesn't mean that it's not getting done. Remember they don't have the fast paced life style that most of us have in the US, so just take a deep breath and it will taken care of. If you feel you need to say something, just make a simple, polite statement and leave it at that-don't try to pitch an argument because you'll have serious problems and it won't go anywhere...
3) Don't make complaints or boss anyone around, no matter what the situation because they don't really appreciate that (in the US we don't tend to care, but the Europeans don't seem to like that)-just be polite and if someone is going to be bossy to you, just be polite, do what they say and if it's something you're not comfortable with just make an excuse and leave..DO NOT SEEK OUT AN ARGUMENT OR MAKE A SCENE which is something these "ugly Americans" love to do!
4) Try to learn some basic words in the language, at least if you can say hello/goodbye please, thank you that's okay. A lot of these "ugly Americans" tend to think to assume that everyone speaks English and yes a lot of people do, but not everyone and not everyone speaks it well, so if you just learn a few words it will go along way and they will have more respect for you than if you just butt in and just start speaking in English. If I traveled to a country where I didn't speak the language, I always asked them in their language if they spoke English or I'd start out the conversation in their language and if they saw I had trouble, if the person knew it always butted in with English. If you just go in and assume that everyone speaks English, you're going to get a rude reaction because well that's just not right. How would you feel if someone came into your store immediately talking to you in French in the US, just assuming that you do or Spanish for that matter? Just attempt to speak as much as you can and if you have problems be patient-and if there's a misunderstanding, don't panic or freak out, just explain nicely that that was not what you wanted or was saying and just go from there...
5) Be Polite and respectful-in the US we do tend to be informal with people, even with strangers. Always be polite and respectful around people even strangers. Remember your pleases and thank yous and yes ma'ams and no ma'ams, even with people who are being rude to you..it just goes a long way and if someone wants to be less formal with you they'll let you know, but when you first start a conversation with someone or even if you walk into a small store or bakery be polite!
6) Don't assume anything, ever! Don't assume that that clerk speaks English fluently just because his store is right across from the Eiffel Tower, don't assume that that man is rude because he's got infront of you in line or it was your turn to order at the bakery but the woman two people behind you butts in anways, or that that man is really being nice and is being helpful but is really a scam artist looking out to rip you off. Also, never assume that everything is like what it is back home...Remember you're in a different country where the official language is not necessarily English so just remember that the culture, customs and life are going to be different and have room for that, and don't dismiss something just because a dish didn't taste right to you or is just different even if it's something you truly don't like because no, it may not be acceptable at home but that's the way they do things there, and you have to respect that. So keep an open mind, be willing to try new things and "go with the flow" and when a cultural difference does turn up just say, okay that's the way it is and just go with it.
7) Use your city maps and all, but don't stop in the middle of a sidewalk to look at it...many American tourists tend to do this and you don't have the sidewalk space for it, so find a corner or a plaza so that you can sit down in a corner, look at your map or take your pictures so that you don't get in anyone's way.
8) This isn't really about being considered rude, but if you don't want to get a whole lot of attention and be pointed out as an obvious American, don't wear shorts or shirts with logos or brands on them, nor wear your camera around your neck or a weird hat-dress like you normally would if you were out on the town back home-just because you're in a foreign country doesn't mean you have to wear your shorts and your Hawaiian shirt or your Budweiser T-shirt...they will know immediately that you're an American tourist because only Americans wear stuff like that. Also don't wear really loud colors; just bring basic, normal clothes and don't keep your camera, guide book and map out in plain sight-this way it will be harder for someone to know where you're from.
9) Try to be as low-key as possible; don't dress, act, or behave as you wouldn't back home-just because you're in a strange country where you don't understand anything doesn't give you an excuss to be different from how you normally are.
10) Read up on your country's cultural norms, language and food before you go so that this way there are no surprises when you get there and the culture shock isn't that overwhelming-also read up on what you want to see too so that you actually can understand and appreciate it.
I hope this all helps but I think if you just keep these in mind you should be fine. But just remember, use common sense and just be a good person...remember you are an unofficial ambassador for your country (though remember just because you are a citizen of a certain country doesn't mean you know everything about your country either nor are the people you meet truly representatives of theirs) but still, people are going to see you as the foreigner and how you react will cause people to judge you according to where you're from, even though it isn't right but treat others as you'd like to be treated, and patiences is a virtue and that should be enough. Good luck, have fun and safe trip!