No flip flops! I know it sucks. They don't take up much space and if it's hot out they're the comfiest thing to wear, but in the countries I've visited (France, Spain, Italy, Germany, Switzerland, etc...) people always wear nice shoes and you stick out like a sore thumb if you don't.

PS! Don't pack a whole lot and leave room in your bag to go shopping. I'm not sure where you'll be travelling, but Europe has some fantastic chain clothing stores that have relatively cheap, stylish clothing. Take note of what people are wearing and try to mimic that. From what I've noticed, the trends are usually a few months ahead of what we wear in the states. Take a chance and get some clothes you wouldn't find back at home and rock them while you're there. In terms of being self conscious as an American, people have always been very nice to me even if they know I'm from the US. I usually identify myself as being from California instead of saying America, so I'm not sure if that makes a difference. People then want to know if you live next to movie stars and surf all day, but at least if gives you something to launch a conversation from. Also, try to pick up some phrases in other languages and even if you feel like an idiot while speaking, try to communicate at first in the language of the country you're in. So many people speak English, but I think they like to see that you're putting in an effort to enjoy and absorb the local culture.