I am a US citizen travelling to the UK for tourism. What kinds of documentation do I need to make sure that I have have with me when I get to passport control to make sure I get a visa on arrival?
You won't get a visa on arrival in the UK but as US citizen you generally don't need a visa. In all likelihood, you won't be asked to show anything but in theory you are supposed to bring the same things that you would need to apply for visa (bank statement, itinerary, etc.).