How can an American traveler not make an a** of themselves while in England?

I plan to visit England for 2 weeks this summer, and I don't want to be such an obvious "American" tourist! How can I prepare myself and what are some things that British people will not tolerate about Americans? I will actually be in the Leeds, Manchester area! Not sure if that will help, but I really would like to make sure I am welcomed back instead of being that typical Yank!