Eating Out (and Tipping) in the US

Americans eat out more than British people, and Americans go out for breakfast more than Brits. Here are some random facts.
Portions are often (not always) larger than you might be used to. It's fine to ask for it to be boxed up to take home.  Every restaurant I've ever been to does take out, so they will have boxes.  It's not weird or embarrassing to ask - it's so normal that if you have a substantial amount left that you haven't eaten, and you don't want to take it home, the wait staff will assume there's a problem.
Tipping. You know this already if you know anything about America. You have to tip. 15% is minimal - that means it was just about OK.  If you go to anywhere regularly it's worth your while to tip at least 20%, and you won't have to go there that regularly to be remembered - you'll be the person with the British accent after all. (In one restaurant near me, I'm the white person with the British accent. I hadn't been in there for about 6 months - I walked in and they said "Hi Jeremy, did you move?"
Soft drinks and coffee will usually be refilled for free.  You keep drinking, and they'll keep filling.  This can lead to a lot of bathroom breaks.