Oddly enough, the saying that came to mind for me is "Ciao!" I don't speak Spanish (though living in California means I've picked up a bit anyhow), but I started saying instead of good-bye at some point back in my university (in Pittsburgh...), and now my friends look at me strangely if I say good-bye. Not as cool as Swedish though :-(
Along the same line, I work in Japan, and at some point when talking I stopped referring to America as the States/the US", and started saying "America" like all the Japanese do. It's weird, especially when actual Japanese words haven't really worn off on me too much.
As for sayings I use a lot, I know where most of them come from. I picked up "No way!" from a co-worker, and "He's just too cool for school" and "I guess that makes me the cock of the walk!" from my old roommate. I don't know anyone else besides him and myself who uses 'cock of the walk' though.