Students from some college down in Tennessee are traveling cross country to ask anyone they happen to meet one question: What does it mean to be an American?
Of course, they get the usual responses ("Freedom! Liberty! Baseball!"). I'm going to throw in my two cents.
I think being an American is to ask oneself what being an American means. The question itself defines an American. It's something that we can only answer ourselves. Our country is constantly looking at itself in close examination eternally trying to determine just what the hell it is supposed to be.
Everyone has a different answer, and these collectively build up into who we are as a nation. We're a nation of 'answers' to this very question. For some, that's 'Life Liberty and Pursuit of Happiness' and for others it's 'maintaining my culture in this heathen country' and for others it's 'get the hell off my lawn, Greg, because you always chuck your goddamn cigarette butts onto it and that's not funny'.
So, there's your answer. If you ponder this question yourself (if you're an American in the first place) and seek out your answer to it, that's what it means to be an American.